contributing: explicitly ban ai-generated stuff

Author: q66

CONTRIBUTING.md

@@ -12,6 +12,10 @@ We suggest to avoid using GitHub's web editor for contributions, especially
 for templates, since those need testing. Therefore, if you do not know at
 least the basics of Git, use some of the public resources to get familiar.
 
+Do not use AI tools to prepare your contribution. Such contributions will
+not be accepted if found, and anyone submitting them will be barred from
+contributing further.
+
 Also read [`USING_GIT.md`](USING_GIT.md) if you are unfamiliar with Git.
 
 Once you have that, clone your fork and prepare the changes.