ci: add markdown link check.

Author: fky2015

Diff for: .github/workflows/test.yaml

@@ -40,3 +40,9 @@ jobs:
             git config --global --add safe.directory /github/workspace
 
             make check-cls
+
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1