add markdown link checker

.github/workflows/action.yml

@@ -0,0 +1,10 @@
+name: Check Markdown links
+on: push
+jobs:
+	markdown-link-check:
+		runs-on: ubuntu-latest
+		steps:
+			- uses: actions/checkout@master
+			- uses: gaurav-nelson/github-action-markdown-link-check@0.6.0
+				with:
+					folder-path: "spec"