feat(CI): add a GitHub Action to lint the Markdown

Add a `.markdownlint.yml` config file

.github/workflows/lint-checker.yml

@@ -26,3 +26,14 @@ jobs:
               echo $diffs
               exit 1
           fi
+  markdownlint:
+    name: 🍇 Markdown
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: 🚀 Use Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: '12.x'
+      - run: npm install -g markdownlint-cli@0.25.0
+      - run: markdownlint '**/*.md' --ignore node_modules

.markdownlint.yml

@@ -0,0 +1,14 @@
+# MD004/ul-style Unordered list style
+MD004: false
+
+# MD013 Line length
+MD013: false
+
+#MD029/ol-prefix Ordered list item prefix
+MD029: false
+
+# MD033 Inline HTML
+MD033: false
+
+# MD036/no-emphasis-as-heading/no-emphasis-as-header Emphasis used instead of a heading
+MD036: false