Add action to show a message when merging to the main branch (#4301)

* Add action to show a message when merging to the main branch

* Fail if label 'merge-main-ack' is not present

.github/workflows/merge-to-main.yml

@@ -0,0 +1,40 @@
+name: Merge to main
+
+on:
+  pull_request:
+    branches:
+      - master
+    types:
+      - opened
+      - labeled
+      - unlabeled
+
+jobs:
+  pr-message:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+    - uses: mshick/add-pr-comment@v2
+      with:
+        message: >
+          ### 📝 PRs merging into main branch
+
+          **Our main branch should always be in a releasable state**.
+          If you are working on a larger change, or if you don't want
+          this change to see the light of the day just yet, consider
+          using a feature branch first, and only merge into the main
+          branch when the code complete and ready to be released.
+
+
+          **Add the 'main-merge-ack' label to your PR to confirm
+          merging into the main branch is intended.**
+
+    - name: Label checker
+      if: "!contains( github.event.pull_request.labels.*.name, 'main-merge-ack')"
+      run: |
+        echo Missing 'main-merge-ack' label. Read the comment about merging to master in your PR for more information.
+        exit 1
+
+    - name: Success
+      run: exit 0