Add example usage

Author: Friendseeker

README.md

@@ -1,2 +1,21 @@
 # scala-cla
 Checks if the author has signed the Scala CLA
+
+Usage
+-----
+
+Here's an example that checks if author of Pull Requests had signed Scala CLA.
+
+```yaml
+name: "Check Scala CLA"
+on:
+  pull_request:
+jobs:
+  cla-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Verify CLA
+        uses: scala/scala-cla
+        with:
+          author: ${{ github.event.pull_request.user.login }}
+```