Add "PR Bad Target" workflow action

.github/workflows/bad-target.md

@@ -0,0 +1,7 @@
+Thanks for your contribution! Unfortunately we can't accept PRs directed at release branches. We make patches to the bugfix branches and only later do we push them out as releases.
+
+Please redo this PR starting with the `bugfix-2.0.x` branch and be careful to target `bugfix-2.0.x` when resubmitting the PR.
+
+It may help to set your fork's default branch to `bugfix-2.0.x`.
+
+See [this page](http://marlinfw.org/docs/development/getting_started_pull_requests.html) for full instructions.

.github/workflows/check-pr.yml

@@ -0,0 +1,27 @@
+#
+# comment-pr.yml
+# Add a comment to any PR directed to a release branch
+#
+
+name: PR Bad Target
+
+on:
+  pull_request:
+    branches:
+    - 1.0.x
+    - 1.1.x
+    - 2.0.x
+
+jobs:
+  bad_target:
+
+    name: PR Bad Target
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: harupy/comment-on-pr@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          filename: bad-target.md