ci: Add Label Commenter Action

https://github.com/peaceiris/actions-label-commenter

.github/workflows/label-commenter.yml

@@ -0,0 +1,25 @@
+name: Label Commenter
+
+on:
+  issues:
+    types:
+      - labeled
+      - unlabeled
+  # pull_request:
+  #   types:
+  #     - labeled
+  #     - unlabeled
+
+jobs:
+  comment:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: master
+
+      - name: Label Commenter
+        uses: peaceiris/actions-label-commenter@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          # config_file: .github/label-commenter-config.yml