CI: check Markdownlint passes

.github/workflows/markdownlint.yml

@@ -0,0 +1,18 @@
+name: Markdown Lint
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  readme-lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install markdownlint
+        run: npm install -g markdownlint-cli
+
+      - name: Lint README
+        run: markdownlint README.md