re-introduce install instructions with to releases (#1649)

Installation instructions were included with releases until 0.16.0, but
lost after that, perhaps with release automation moving to GitHub. These
instructions are still referenced in the README.md as being available in
Releases, so this PR reintroduces them.


**Description of the change**

Re-introduces installation instructions in Releases, as referenced by
README.md

**Benefits**

Maintain consistency with README instructions

**Possible drawbacks**

Splitting the instructions across multiple locations. It may be
preferred to move them to README.md, but automation will be required to
keep version numbers consistent.

**Applicable issues**

<!-- Enter any applicable Issues here (You can reference an issue using
#) -->
- fixes #1648 

**Additional information**

Install instructions were taken from those previously present in
Releases, as well as from existing README instructions for client
installs. There may be additional client installs or a change to the
preferred controller installation method which hasn't been considered
here.

Signed-off-by: Anthony Goddard <[email protected]>

Author: agoddard

.goreleaser.yml

@@ -58,11 +58,73 @@ release:
 
     New v{{ .Version }} release!
   footer: |
-    ## Release Notes
+    ## Installation Instructions
+
+    ### Cluster-side
+
+    Install the SealedSecret CRD and server-side controller into the `kube-system` namespace:
+
+    ```sh
+    kubectl apply -f https://github.com/bitnami-labs/sealed-secrets/releases/download/v{{ .Version }}/controller.yaml
+    ```
+
+    ### Client-side
+
+    Install the client-side tool into `/usr/local/bin/`:
+
+    **Linux x86_64:**
+    ```sh
+    curl -OL "https://github.com/bitnami-labs/sealed-secrets/releases/download/v{{ .Version }}/kubeseal-{{ .Version }}-linux-amd64.tar.gz"
+    tar -xvzf kubeseal-{{ .Version }}-linux-amd64.tar.gz kubeseal
+    sudo install -m 755 kubeseal /usr/local/bin/kubeseal
+    ```
+
+    **macOS:**
+    The `kubeseal` client is available on [homebrew](https://formulae.brew.sh/formula/kubeseal):
+
+    ```sh
+    brew install kubeseal
+    ```
+
+    **MacPorts:**
+
+    The `kubeseal` client is available on [MacPorts](https://ports.macports.org/port/kubeseal/summary):
+
+    ```sh
+    port install kubeseal
+    ```
 
-    Please read the [RELEASE_NOTES](https://github.com/bitnami-labs/sealed-secrets/blob/main/RELEASE-NOTES.md) which contain among other things important information for who is upgrading from previous releases.
+    #### Nixpkgs
 
+    The `kubeseal` client is available on [Nixpkgs](https://search.nixos.org/packages?channel=unstable&show=kubeseal&from=0&size=50&sort=relevance&type=packages&query=kubeseal): (**DISCLAIMER**: Not maintained by bitnami-labs)
+
+    ```sh
+    nix-env -iA nixpkgs.kubeseal
+    ```
+
+    **Other OS/Arch:**
+    Binaries for other OS/arch combinations are attached to this release below.
+
+    If you just want the latest client tool, it can be installed into
+    `$GOPATH/bin` with:
+
+    ```sh
+    go install github.com/bitnami-labs/sealed-secrets/cmd/kubeseal@main
+    ```
+
+    You can specify a release tag or a commit SHA instead of `main`.
+
+    The `go install` command will place the `kubeseal` binary at `$GOPATH/bin`:
+
+    ```sh
+    $(go env GOPATH)/bin/kubeseal
+    ```
+
+    ## Release Notes
+
+    Please read the [RELEASE_NOTES](https://github.com/bitnami-labs/sealed-secrets/blob/main/RELEASE-NOTES.md) which contain among other things important information for those upgrading from previous releases.
     ## Thanks!
+
   extra_files:
     - glob: ./controller.yaml
     - glob: ./controller-norbac.yaml