docs: add gar and gcr example (#128)

README.md

@@ -277,6 +277,49 @@ jobs:
         credentials_json: '${{ secrets.GOOGLE_CREDENTIALS }}'
 ```
 
+### Authenticating to Container Registry and Artifact Registry
+
+This example demonstrates authenticating to Google Container Registry (GCR) or
+Google Artifact Registry (GAR). The most common way to authenticate to these
+services is via a gcloud docker proxy. However, you can authenticate to these
+registries directly using the `auth` action:
+
+-   **Username:** `oauth2accesstoken`
+-   **Password:** `${{ steps.auth.outputs.access_token }}`
+
+You must set `token_format: access_token` in your Action YAML. Here are a few
+examples:
+
+```yaml
+jobs:
+  job_id:
+    steps:
+    - uses: 'actions/checkout@v2'
+
+    - id: 'auth'
+      name: 'Authenticate to Google Cloud'
+      uses: 'google-github-actions/auth@v0'
+      with:
+        token_format: 'access_token'
+        # Either user Workload Identity Federation or Service Account Keys. See
+        # above more more examples
+
+    # This example uses the docker login action
+    - uses: 'docker/login-action@v1'
+      with:
+        registry: 'gcr.io' # or REGION.docker.pkg.dev
+        username: 'oauth2accesstoken'
+        password: '${{ steps.auth.outputs.access_token }}'
+
+    # This example runs "docker login" directly to Artifact Registry.
+    - run: |-
+        echo '${{ steps.auth.outputs.access_token }}' | docker login -u oauth2accesstoken --password-stdin https://REGION-docker.pkg.dev
+
+    # This example runs "docker login" directly to Container Registry.
+    - run: |-
+        echo '${{ steps.auth.outputs.access_token }}' | docker login -u oauth2accesstoken --password-stdin https://gcr.io
+```
+
 ### Configuring gcloud
 
 This example demonstrates using this GitHub Action to configure authentication