Add github action to find HTTP resources returning a 404 error code

Author: nehhen

.github/workflows/markdown-lint.yml

@@ -0,0 +1,17 @@
+name: Markdown lint
+# https://github.com/marketplace/actions/404-links
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    types: [assigned, opened, synchronize, reopened]
+
+jobs:
+  check-links:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: 'restqa-404-links'
+      uses: restqa/404-links@3.1.4
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}