Add extension gallery update reminder action (#17644)

.github/workflows/update-galleries-list-reminder.yml

@@ -0,0 +1,20 @@
+name: Update Extension Gallery Reminder
+on:
+  pull_request:
+    paths:
+      - 'extensionGallery.json'
+    branches:
+      - release/extensions
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: mshick/add-pr-comment@v1
+        with:
+          message: |
+            This is a reminder to update the Extensions Gallery list at https://github.com/Microsoft/azuredatastudio/wiki/List-of-Extensions once this PR has been merged in.
+            Instructions for doing this can be found [here](https://github.com/microsoft/azuredatastudio-docs/blob/main/Guides/UpdateExtensionGallery.md#guide-for-extension-release-dri).
+
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          repo-token-user-login: 'github-actions[bot]' # The user.login for temporary GitHub tokens