chore: update readme for gcp resource detector

Update README of GCP Resource Detector to clarify the source of the data.

Part Of #2055

Signed-off-by: maryliag <[email protected]>

Author: maryliag

detectors/node/opentelemetry-resource-detector-gcp/README.md

@@ -27,6 +27,22 @@ const resource = await detectResources({
 const tracerProvider = new NodeTracerProvider({ resource });
 ```
 
+## Available detectors
+
+### GCP Detector
+
+| Resource Attribute      | Description                                                   |
+|-------------------------|---------------------------------------------------------------|
+| cloud.account.id        | Value of `project-id` from GCP Metadata project               |
+| cloud.availability_zone | Value of `zone` from GCP Metadata instance                    |
+| cloud.provider          | The cloud provider. In this context, it's always "gcp"        |
+| container.name          | Value of Environment Variable `CONTAINER_NAME`                |
+| host.id                 | Value of `id` from GCP Metadata instance                      |
+| host.name               | Value of `hostname` from GCP Metadata instance                |
+| k8s.cluster.name        | Value of `attributes/cluster-name` from GCP Metadata instance |
+| k8s.namespace.name      | Value of Environment Variable `NAMESPACE`                     |
+| k8s.pod.name            | Value of Environment Variable `HOSTNAME`                      |
+
 ## Useful links
 
 - [GCP Metadata Documentation][]