docs(extensions): add documentation for extensions (#340)

Adds a new documentation page for Gemini CLI extensions, explaining how
to configure and use them. The main README is updated to include a link
to the new documentation.

This is a follow up after
#329.

Author: jerop

README.md

@@ -20,6 +20,7 @@ Use it to perform GitHub pull request reviews, triage issues, perform code analy
     - [Issue Triage](#issue-triage)
     - [Pull Request Review](#pull-request-review)
     - [Gemini CLI Assistant](#gemini-cli-assistant)
+  - [Configuration](#configuration)
     - [Inputs](#inputs)
     - [Outputs](#outputs)
     - [Repository Variables](#repository-variables)
@@ -28,6 +29,7 @@ Use it to perform GitHub pull request reviews, triage issues, perform code analy
     - [Google Authentication](#google-authentication)
     - [GitHub Authentication](#github-authentication)
   - [Observability](#observability)
+  - [Extensions](#extensions)
   - [Best Practices](#best-practices)
   - [Customization](#customization)
   - [Contributing](#contributing)
@@ -137,15 +139,15 @@ This action can be used to automatically review pull requests when they are
 opened. For a detailed guide on how to set up the pull request review system,
 go to the [GitHub PR Review workflow documentation](./examples/workflows/pr-review).
 
-
-
 ### Gemini CLI Assistant
 
 This type of action can be used to invoke a general-purpose, conversational Gemini
 AI assistant within the pull requests and issues to perform a wide range of
 tasks. For a detailed guide on how to set up the general-purpose Gemini CLI workflow,
 go to the [Gemini Assistant workflow documentation](./examples/workflows/gemini-assistant).
 
+## Configuration
+
 ### Inputs
 
 <!-- BEGIN_AUTOGEN_INPUTS -->
@@ -271,6 +273,14 @@ for debugging and optimization.
 For detailed instructions on how to set up and configure observability, go to
 the [Observability documentation](./docs/observability.md).
 
+## Extensions
+
+The Gemini CLI can be extended with additional functionality through extensions.
+These extensions are installed from source from their GitHub repositories.
+
+For detailed instructions on how to set up and configure extensions, go to the
+[Extensions documentation](./docs/extensions.md).
+
 ## Best Practices
 
 To ensure the security, reliability, and efficiency of your automated workflows, we strongly recommend following our best practices. These guidelines cover key areas such as repository security, workflow configuration, and monitoring.

docs/extensions.md

@@ -0,0 +1,34 @@
+# Extensions
+
+Gemini CLI can be extended with additional functionality through extensions.
+These extensions are installed from source from their GitHub repositories.
+
+For more information on creating and using extensions, see [documentation].
+
+[documentation]: https://github.com/google-gemini/gemini-cli/blob/main/docs/extensions/index.md
+
+## Configuration
+
+To use extensions in your GitHub workflow, provide a JSON array of GitHub
+repositories to the `extensions` input of the `run-gemini-cli` action.
+
+### Example
+
+Here is an example of how to configure a workflow to install and use extensions:
+
+```yaml
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - id: gemini
+        uses: google-github-actions/run-gemini-cli@v0
+        with:
+          gemini_api_key: ${{ secrets.GEMINI_API_KEY }}
+          prompt: "/security:analyze"
+          extensions: |
+            [
+              "https://github.com/gemini-cli-extensions/security",
+              "https://github.com/gemini-cli-extensions/code-review"
+            ]
+```