Add GitHub Workflow to mirror master and main branches

.github/workflows/mirror-master-and-main.yml

@@ -0,0 +1,19 @@
+name: Mirror "master" and "main" branches
+on:
+  push:
+    branches:
+      - master
+      - main
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Push
+        run: |
+          git push origin HEAD:master
+          git push origin HEAD:main