Add action to warn against PR against master (should be develop)

.github/workflows/master_branch_pr.yml

@@ -0,0 +1,18 @@
+on:
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post PR comment
+        if: failure()
+        uses: mshick/add-pr-comment@v1
+        with:
+          message: |
+            Hi @${{ github.event.pull_request.user.login }},
+
+            It looks like this pull request has been made against the ${{github.event.pull_request.head.repo.full_name}} `master` branch.
+            Since we use Git Flow all commits to master are through are from the develop branch.
+            You do not need to close this PR, you can change the target branch to `development` by clicking the _"Edit"_ button at the top of this page.