Add an action for proper repository mirroring

Do roughly the same with your repositories if you have exactly the same
copies of the repositories on other forges (GitLab, Codeberg, etc.).

:^)

.github/workflows/codeberg-mirror.yml

@@ -0,0 +1,17 @@
+name: Codeberg Mirroring
+
+on:
+  push:
+    branches: ["master"]
+
+jobs:
+  to_codeberg:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: pixta-dev/repository-mirroring-action@v1
+        with:
+          target_repo_url: "git@codeberg.org:openvk/openvk.git"
+          ssh_private_key: ${{ secrets.CODEBERG_PRIVSSH }}