fix: update docs (#588)

README.md

@@ -153,6 +153,45 @@ job:
       run: 'gcloud info'
 ```
 
+### Multiple Service Accounts
+
+To use multiple service accounts, a second auth step is required to update the credentials before using `setup-gcloud`:
+
+```yaml
+jobs:
+  job_id:
+    # Add "id-token" with the intended permissions.
+    permissions:
+      contents: 'read'
+      id-token: 'write'
+
+    steps:
+      - id: 'auth service account 1'
+        uses: 'google-github-actions/auth@v1'
+        with:
+          workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
+          service_account: 'service-account-1@my-project.iam.gserviceaccount.com'
+
+      - name: 'Set up Cloud SDK'
+        uses: 'google-github-actions/setup-gcloud@v1'
+
+      - name: 'Use gcloud CLI'
+        run: 'gcloud auth list --filter=status:ACTIVE --format="value(account)"'
+        # service-account-1@my-project.iam.gserviceaccount.com
+
+      - id: 'auth service account 2'
+        uses: 'google-github-actions/auth@v1'
+        with:
+          credentials_json: '${{ secrets.GCP_CREDENTIALS }}'
+
+      - name: 'Set up Cloud SDK'
+        uses: 'google-github-actions/setup-gcloud@v1'
+
+      - name: 'Use gcloud CLI'
+        run: 'gcloud auth list --filter=status:ACTIVE --format="value(account)"'
+        # service-account-2@my-project.iam.gserviceaccount.com
+```
+
 
 ## Versioning