Create workflow to mirror to GitLab (#52)

Author: mirand863

.github/workflows/mirror-gitlab

@@ -0,0 +1,17 @@
+name: Mirror to GitLab
+
+on: [push, delete]
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:                                              # <-- must use actions/checkout before mirroring!
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: pixta-dev/repository-mirroring-action@v1
+        with:
+          target_repo_url:
+            [email protected]:dacs-hpi/hiclass.git
+          ssh_private_key:                              # <-- use 'secrets' to pass credential information.
+            ${{ secrets.GITLAB_SSH_PRIVATE_KEY }}