docs: add GHA config for the deployment API (#8983)

docs/src/ci.md

@@ -76,6 +76,36 @@ steps:
 
 We run [our tests](https://github.com/microsoft/playwright/blob/master/.github/workflows/tests_secondary.yml) on GitHub Actions, across a matrix of 3 platforms (Windows, Linux, macOS) and 3 browsers (Chromium, Firefox, WebKit).
 
+### GitHub Actions on deployment
+
+This will start the tests after a [GitHub Deployment](https://developer.github.com/v3/repos/deployments/) went into the `success` state.
+Services like Azure Static Web Apps, Netlify, Vercel, etc. use this pattern so you can run your end-to-end tests on their deployed enviornment.
+
+```yml
+name: Playwright Tests
+on:
+  deployment_status:
+jobs:
+  test:
+    timeout-minutes: 60
+    runs-on: ubuntu-latest
+    if: github.event.deployment_status.state == 'success'
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-node@v2
+      with:
+        node-version: '14.x'
+    - name: Install dependencies
+      run: npm ci
+    - name: Install Playwright
+      run: npx playwright install --with-deps
+    - name: Run Playwright tests
+      run: npm run test:e2e
+      env:
+        # This might depend on your test-runner/language binding
+        PLAYWRIGHT_TEST_BASE_URL: ${{ github.event.deployment_status.target_url }}
+```
+
 ### Docker
 
 We have a [pre-built Docker image](./docker.md) which can either be used directly, or as a reference to update your existing Docker definitions.