add a bot for eslint on github

.github/workflows/reviewdog.yml

@@ -0,0 +1,12 @@
+name: reviewdog
+on: [pull_request]
+jobs:
+  eslint:
+    name: runner / eslint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: reviewdog/action-eslint@v1
+        with:
+          reporter: github-pr-review # Change reporter.
+          eslint_flags: 'src/'