Add Commit Signing section (#2019)

* Add Commit Signing section

* Clarify that this only affects to internal branches

Co-authored-by: Julian Gonzalez Calderon <[email protected]>

---------

Co-authored-by: Julian Gonzalez Calderon <[email protected]>
Co-authored-by: Franco Giachetta <[email protected]>

Author: 3 people

CONTRIBUTING.md

@@ -110,6 +110,9 @@ Enhancement suggestions are tracked as [GitHub issues](https://github.com/lambda
 - You may want to **include screenshots and animated GIFs** which help you demonstrate the steps or point out the part which the suggestion is related to. You can use [this tool](https://www.cockos.com/licecap/) to record GIFs on macOS and Windows, and [this tool](https://github.com/colinkeenan/silentcast) or [this tool](https://github.com/GNOME/byzanz) on Linux. 
 - **Explain why this enhancement would be useful** to most cairo-vm users. You may also want to point out the other projects that solved it better and which could serve as inspiration.
 
+### Pushing commits to Pull Requests
+
+This repository enforces commit signing on all branches. See [this page](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits) for more details on how to sign a commit and how to manage signing keys in Github.
 
 <!-- TODO
 ### Your First Code Contribution