Add DCO signoff ref to contributing doc

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

CONTRIBUTING.md

 # Contributing
 
-Flux is [Apache 2.0
-licensed](https://github.com/fluxcd/flux2/blob/main/LICENSE) and
+Flux is [Apache 2.0 licensed](https://github.com/fluxcd/flux2/blob/main/LICENSE) and
 accepts contributions via GitHub pull requests. This document outlines
 some of the conventions on to make it easier to get your contribution
 accepted.
@@ -14,9 +13,18 @@ code.
 By contributing to this project you agree to the Developer Certificate of
 Origin (DCO). This document was created by the Linux Kernel community and is a
 simple statement that you, as a contributor, have the legal right to make the
-contribution. No action from you is required, but it's a good idea to see the
-[DCO](DCO) file for details before you start contributing code to FluxCD
-organization.
+contribution.
+
+We require all commits to be signed. By signing off with your signature, you
+certify that you wrote the patch or otherwise have the right to contribute the
+material by the rules of the [DCO](DCO):
+
+`Signed-off-by: Jane Doe <jane.doe@example.com>`
+
+The signature must contain your real name
+(sorry, no pseudonyms or anonymous contributions)
+If your `user.name` and `user.email` are configured in your Git config,
+you can sign your commit automatically with `git commit -s`.
 
 ## Communications