github: add action to automatically close pull requests.

Change-Id: Icb1164879c9bd0a48aad21d7a42cba79ab195677
Reviewed-on: https://code.wireshark.org/review/36353
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

.github/workflows/close_pr.yml

@@ -0,0 +1,22 @@
+name: Close Pull Requests
+
+on:
+  pull_request:
+    types: [opened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v2
+      with:
+        # Optional. Post a issue comment just before closing a pull request.
+        comment: |
+          Hi, thank you for your contribution!
+          Github is however not the right place for these, please have a look at
+
+          https://www.wireshark.org/docs/wsdg_html_chunked/ChSrcContribute.html
+
+          for further instructions.
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}