Mention that public contribution commits need to be signed (#1287)

CONTRIBUTING.md

@@ -11,6 +11,9 @@ We aim to follow high quality standards, thus your PR must follow some rules:
 - Make sure your changes are compatible (or protected) with older Kubernetes version (CI will validate this down to 1.14)
 - Make sure you updated documentation (after bumping `Chart.yaml`) by running `.github/helm-docs.sh`
 
+Additionally, your commits need to be signed and marked as verified by Github. See [About commit signature verification
+](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification).
+
 Our team will then happily review and merge contributions!
 
 ## Go Tests
@@ -113,4 +116,4 @@ In each chart, the `README.md` file is generated from the corresponding `README.
 [pulumi]:https://www.pulumi.com/
 [test-infra-repo]:https://github.com/DataDog/test-infra-definitions
 [agent-e2e-source]:https://github.com/DataDog/datadog-agent/tree/main/test/new-e2e
-[test-infra-quickstart]:https://github.com/DataDog/test-infra-definitions#quick-start-guide
+[test-infra-quickstart]:https://github.com/DataDog/test-infra-definitions#quick-start-guide