Add link checker

.github/workflows/check-links.yaml

@@ -0,0 +1,15 @@
+name: Check Markdown links
+
+on:
+  workflow_dispatch:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 0 1 * *"
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - uses: gaurav-nelson/github-action-markdown-link-check@v1