Merge pull request #882 from smallrye/document-commit-requirements

Document the Git commit requirements

CONTRIBUTING.md

@@ -27,6 +27,12 @@ Don't forget to indicate your SmallRye Mutiny, Java, and Maven/Gradle version.
 
 To contribute, use GitHub Pull Requests, from your **own** fork.
 
+### Signing your Git commits
+
+We require Git commits to be *[verified on GitHub](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification)*, so please:
+- [use a valid user name and email in your commits](https://docs.github.com/en/account-and-profile/setting-up-and-managing-your-github-user-account/managing-email-preferences/setting-your-commit-email-address), and
+- [sign your commits](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits) with a GnuPG key that is tied to your GitHub account. 
+
 ### Code reviews
 
 All non-trivial contributions, including contributions by project members, need to be reviewed before being merged.