Add Github CI templated and workflow.

.github/workflows/check-pr.yml

@@ -0,0 +1,31 @@
+#
+# check-pr.yml
+# Close PRs directed at release branches
+#
+
+name: PR Bad Target
+
+on:
+  pull_request:
+    branches:
+    - 2.0.x
+
+jobs:
+  bad_target:
+    name: PR Bad Target
+    if: github.repository == 'knutwurst/Marlin-2-0-x-Anycubic-i3-MEGA-S'
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: peter-evans/close-pull@v1
+      with:
+        delete-branch: false
+        comment: >
+          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.