Add README for k8s multi cluster plugin

pkg/app/pipedv1/plugin/kubernetes_multicluster/README.md

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+# Kubernetes Multicluster Plugin
+
+## Specification
+
+The current specification is described in the [RFC](../../../../../docs/rfcs/0014-multi-cluster-deployment-for-k8s.md).  
+The configuration format is unstable and may change in the future.
+
+## Try k8s multicluster plugin locally
+
+**Prepare the PipeCD Control Plane**
+
+Please refer to [pipe-cd/pipecd/cmd/pipecd/README.md](../../../../../cmd/pipecd/README.md) to set up the Control Plane in your local environment.
+
+**Prepare two k8s clusters**
+
+```sh
+kind create cluster --name cluster1
+kind export kubeconfig --name cluster1 --kubeconfig /path/to/kubeconfig/for/cluster1
+
+kind create cluster --name cluster2
+kind export kubeconfig --name cluster2 --kubeconfig /path/to/kubeconfig/for/cluster2
+```
+
+**Start pipedv1 locally**
+
+Please refer to [pipe-cd/pipecd/cmd/pipedv1/README.md](../../../../../cmd/pipedv1/README.md) to set up the Control Plane in your local environment.  
+At this time, please modify the `spec.plugins` section of the piped config as shown below.
+
+```yaml
+apiVersion: pipecd.dev/v1beta1
+kind: Piped
+spec:
+  ...
+  plugins:
+  - name: kubernetes_multicluster
+    port: 7002 # any unused port
+    url: file:///path/to/.piped/plugins/kubernetes_multicluster # It's OK using any value for now because it's a dummy. We will implement it later.
+    deployTargets: 
+    - name: cluster1
+      config:
+        masterURL: https://127.0.0.1:61337   # shown by kubectl cluster-info
+        kubeConfigPath: /path/to/kubeconfig/for/cluster1
+    - name: cluster2
+      config:
+        masterURL: https://127.0.0.1:62082   # shown by kubectl cluster-info
+        kubeConfigPath: /path/to/kubeconfig/for/cluster2
+```
+
+**Prepare the manifest**
+
+- Please create a new repository for the manifest because an error will occur if there is an app.pipecd.yaml in the format before supporting the plugin mechanism for now.
+For example usage, see the [Examples](#examples) section below.
+
+**Register the application**
+
+![adding-application](./docs/static/adding-application.png)
+
+At this time, please select multiple DeployTargets.
+
+## Examples
+There are examples under `./example`.
+
+| Name | Description |
+|------|-------------|
+| [simple](./example/simple/) | Deploy the same resources to the multiple clusters. |
+| [multi-sources-template-none](./example/multi-sources-template-none/) | Deploy the different resources to the multiple clusters. |
+
+## Config Reference
+
+### Piped Config
+
+```yaml
+apiVersion: pipecd.dev/v1beta1
+kind: Piped
+spec:
+  ...
+  plugins:
+    # Plugin name.
+  - name: kubernetes_multicluster
+    # Port number used to listen plugin server.
+    port: 7002
+    # The URL where the plugin binary located.
+    # It's OK using any value for now because it's a dummy. We will implement it later.
+    url: file:///path/to/.piped/plugins/kubernetes_multicluster
+    # List of the information for each target platform.
+    # This is alternative for the platform providers.
+    deployTargets: 
+      # Then name of deploy target.
+    - name: cluster1
+      # The plugin-specific config.
+      config:
+        # The master URL of the kubernetes cluster.
+        # Empty means in-cluster.
+        masterURL: https://127.0.0.1:61337
+        # The path to the kubeconfig file.
+        # Empty means in-cluster.
+        kubeConfigPath: /path/to/kubeconfig/for/cluster1
+        # Version of kubectl will be used.
+        kubectlVersion: 1.32.0
+    - name: cluster2
+      config:
+        masterURL: https://127.0.0.1:62082
+        kubeConfigPath: /path/to/kubeconfig/for/cluster2
+```
+
+### app.pipecd.yaml
+
+```yaml
+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  ...
+  plugins:
+    kubernetes_multicluster:
+      input:
+        # List of manifest files in the application directory used to deploy.
+        # Empty means all manifest files in the directory will be used.
+        manifests: 
+          - deployment.yaml
+          - service.yaml
+        # Version of kubectl will be used.
+        kubectlVersion: 1.32.0
+        # The namespace where manifests will be applied.
+        namespace: example
+        # Automatically create a new namespace if it does not exist.
+        # Default is false.
+        autoCreateNamespace: false
+        # List of the setting for each deploy targets.
+        # You can also set config for them separately.
+        multiTargets:
+            # The identity of the deploy target.
+            # You can specify deploy target by some of them.
+          - target:
+              # The name of deploy target
+              name: cluster1
+            # List of manifest files in the application directory used to deploy.
+            # Empty means all manifest files in the directory will be used.
+            manifests:
+              - ./cluster1/deployment.yaml
+            # Version of kubectl will be used.
+            kubectlVersion: 1.32.2
+          - target:
+              name: cluster2
+            manifests:
+              - ./cluster2/deployment.yaml
+            kubectlVersion: 1.32.2
+```

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/multi-sources-template-none/app.pipecd.yaml

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+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  name: multi-sources-template-none
+  labels:
+    env: example
+  plugins:
+    kubernetes_multicluster:
+      quickSync:
+        prune: true
+      input:
+        multiTargets:
+          - target:
+              name: cluster1
+            manifests:
+              - ./cluster1/deployment.yaml
+          - target:
+              name: cluster2
+            manifests:
+              - ./cluster2/deployment.yaml
+        kubectlVersion: 1.32.2

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/multi-sources-template-none/cluster1/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: cluster1
+  labels:
+    app: cluster1
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app: cluster1
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: cluster1
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/multi-sources-template-none/cluster2/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: cluster2
+  labels:
+    app: cluster2
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app: cluster2
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: cluster2
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/simple/app.pipecd.yaml

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+apiVersion: pipecd.dev/v1beta1
+kind: Application
+spec:
+  name: simple
+  labels:
+    env: example
+  plugins:
+    kubernetes_multicluster:
+      input:
+        manifests:
+          - deployment.yaml
+          - service.yaml
+        kubectlVersion: 1.32.2

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/simple/deployment.yaml

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+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: simple
+  labels:
+    app: simple
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app: simple
+      pipecd.dev/variant: primary
+  template:
+    metadata:
+      labels:
+        app: simple
+        pipecd.dev/variant: primary
+      annotations:
+        sidecar.istio.io/inject: "false"
+    spec:
+      containers:
+      - name: helloworld
+        image: ghcr.io/pipe-cd/helloworld:v0.32.0
+        args:
+          - server
+        ports:
+        - containerPort: 9085

pkg/app/pipedv1/plugin/kubernetes_multicluster/example/simple/service.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: simple
+spec:
+  selector:
+    app: simple
+  ports:
+    - protocol: TCP
+      port: 9085
+      targetPort: 9085