apply yamllint on CI

Author: rhysd

.github/workflows/ci.yml

@@ -180,3 +180,7 @@ jobs:
       - run: npm run lint
       - run: npm run build
       - run: npm test
+      - name: Apply yamllint
+        run: |
+          sudo pip install yamllint
+          yamllint .github/workflows

.yamllint.yml

@@ -0,0 +1,6 @@
+extends: default
+
+rules:
+  line-length: disable
+  document-start: disable
+  truthy: disable