ci: Add commitlint GitHub workflow

commitlint [1] will validate the commit message when a pull request is created. A GitHub action was added to the repository, see [2].

[1] https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional
[2] https://github.com/wagoid/commitlint-github-action

.github/workflows/commitlint.yml

@@ -0,0 +1,11 @@
+name: Lint Commit Messages
+on: [pull_request]
+
+jobs:
+  commitlint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - uses: wagoid/commitlint-github-action@v2