Add example.yml, link, and glob syntax to README.md.

.github/workflows/example.yml

@@ -0,0 +1,12 @@
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: DavidAnson/markdownlint-cli2-action@v1
+      with:
+        globs: |
+          *.md
+          !test/*.md