ci(mdl): add markdown linter

.github/workflows/markdownlint.yaml

@@ -0,0 +1,10 @@
+name: markdownlint
+on: [push, pull_request]
+jobs:
+  delivery:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Check out code
+      uses: actions/checkout@master
+    - name: Run mdl (with rules)
+      uses: wopian/markdownlint@master