Add label commenter workflow

Right now the main purpose is to handle instructions for stable
backports. Can be expanded in the future.

.github/workflows/label-commenter.yml

@@ -0,0 +1,22 @@
+name: Label Commenter
+
+on:
+  issues:
+    types:
+      - labeled
+      - unlabeled
+  pull_request_target:
+    types:
+      - labeled
+      - unlabeled
+
+jobs:
+  comment:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: master
+
+      - name: Label Commenter
+        uses: peaceiris/actions-label-commenter@v1