Add CNCF CLA to the contributing document (#4281)

CONTRIBUTING.md

@@ -7,4 +7,5 @@
 1. Submit an issue describing your proposed change to the repo in question.
 2. The [repo owners](OWNERS) will respond to your issue promptly.
 3. Fork the desired repo, develop and test your code changes.
-4. Submit a pull request.
+4. Sign the CNCF CLA (https://git.k8s.io/community/CLA.md#the-contributor-license-agreement)
+5. Submit a pull request.