Making clear that AI-generated PRs are not allowed (openhwgroup#3000)

This follows PRs from @ibooga generated by Claude Code that spam the reviewers.
We need to find a technical solution to instruct language modes or AI coding assistants not to submit PRs.
In the meantime, the rule is written in CONTRIBUTING.md.

Author: jquevremont

CONTRIBUTING.md

@@ -62,6 +62,10 @@ If you encounter difficulties with these guidelines, get in touch with the team!
 
 Bug fixing is always welcome. You can issue a GitHub issue. Better: solve the bug and trigger a pull request.
 
+### Use of AI
+
+We understand that some developers are assisted by large language models to help write code. AI-generated code shall always be reviewed by a human intelligence. However, automated pull requests directly generated by large language models or AI coding assistants are not permitted in this repository.
+
 ## The Mechanics
 1. From GitHub: [fork](https://help.github.com/articles/fork-a-repo/) the [cva6](https://github.com/openhwgroup/cva6) repository
 2. Clone repository: `git clone https://github.com/[your_github_username]/cva6`