docs: update CONTRIBUTING with a note about vibe-coding

CONTRIBUTING.md

@@ -17,14 +17,15 @@ First, please check someone didn't suggest your next revolution in the _Issues_
 
 ## Writing code
 
-[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/Kosmorro/lib)
-
 First of all, if you are fixing an opened issue, check that nobody is already working on it — if someone seems to be but their Pull Request seems stuck, please ask them first if you can continue the development. If you retake the code they produced, **don't change the author of the commits**.
 
 Before writing the code, first create a fork of the repository and clone it. You may also want to add the original repository (`Deuchnord/kosmorro`), so you can update your fork with the last upstream commits.
 
 Then create a new branch and start coding. Finally, commit and push, then open a PR on this project. If your project is not complete, feel free to open it as Draft (if you forgot to activate the Draft status, just edit the first comment to say it), then mark it as ready for review when you're done.
 
+> [!warning]
+> LLM-generated code is not accepted in contributions.
+
 ### Choosing the right branch
 
 When you create your PR, please select the right base and target branch depending on what you are doing: