Add example workflow demonstrating linting only changed files (closes #12).

.github/workflows/changed.yml

@@ -0,0 +1,18 @@
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        fetch-depth: 2
+    - uses: tj-actions/changed-files@v24
+      id: changed-files
+      with:
+        files: '**/*.md'
+        separator: "\n"
+    - uses: DavidAnson/markdownlint-cli2-action@v6
+      if: steps.changed-files.outputs.any_changed == 'true'
+      with:
+        globs: ${{ steps.changed-files.outputs.all_changed_files }}