CONTRIBUTING.md: Ask folks to disclose AI-written PRs (#58666)

Author: DilumAluthge

CONTRIBUTING.md

@@ -54,6 +54,8 @@ A useful bug report filed as a GitHub issue provides information about how to re
 
 * Review discussions on the [Julia Discourse forum](https://discourse.julialang.org).
 
+* If your pull request contains substantial contributions from a generative AI tool, please disclose so with details, and review all changes before opening.
+
 * Relax and have fun!
 
 ### Guidance for specific changes