Document usage in README

README.md

@@ -0,0 +1,30 @@
+Composer Rich Diff Action
+=========================
+
+This is a GitHub Action which runs against a pull request and generates a comment
+documenting the changes to Composer dependencies that are included in the pull
+request.
+
+Usage
+-----
+
+```yaml
+name: Composer Rich Diff
+
+on:
+  pull_request:
+
+permissions:
+    contents: read
+    pull-requests: write
+
+jobs:
+  diff:
+    name: Generate Diff
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cs278/composer-rich-diff-action@v1
+        with:
+          path: ./composer.json
+```