github: add PR closer

We don't accept PRs via GH, so add a job to automatically close them
with an explanation for how to submit.

Change-Id: I5cc3176549a04ff23b04dae1110cd27a58ba1fd3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/386134
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

.github/workflows/close-pull-request.yml

@@ -0,0 +1,22 @@
+# GitHub actions workflow.
+# https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions
+
+# https://github.com/superbrothers/close-pull-request
+name: Close Pull Request
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v3
+      with:
+        comment: >
+          Thanks for your contribution!
+          Unfortunately, we don't use GitHub pull requests to manage code
+          contributions to this repository.
+          Instead, please see [README.md](../blob/HEAD/SUBMITTING_PATCHES.md)
+          which provides full instructions on how to get involved.