Add docs and tests for plugins setup

Signed-off-by: Matheus Pimenta <matheuscscp@gmail.com>

action/README.md

@@ -12,6 +12,25 @@ steps:
     run: flux version --client
 ```
 
+To install Flux plugins alongside the CLI:
+
+```yaml
+steps:
+  - name: Setup Flux CLI
+    uses: fluxcd/flux2/action@main
+    with:
+      plugins: |
+        mirror@0.0.1
+  - name: Run Flux plugin
+    run: flux mirror --help
+```
+
+Installing plugins requires a Flux version with plugin support (v2.9.0 or later).
+The `plugins` input accepts one plugin per line in `<name>@<version|digest>`
+format. The `plugin-dir` input is only used when `plugins` is set; when
+plugins are installed, the action exports `FLUXCD_PLUGINS` for subsequent
+steps.
+
 The Flux GitHub Action can be used to automate various tasks in CI, such as:
 
 - [Automate Flux upgrades on clusters via Pull Requests](https://fluxcd.io/flux/flux-gh-action/#automate-flux-updates)