Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -22,6 +22,8 @@ Contributions are welcomed and encouraged. If you're thinking of fixing a bug or
 
 * Please follow the existing coding conventions, and ensure that there are no linting errors (`npm run lint`). The eslint config doesn't specify all our coding conventions, so please try to be consistent.
 
+* Code or documentation contributions produced by generative AI tools are not allowed.
+
 * For commit messages, please follow [these guidelines](https://chris.beams.io/posts/git-commit/), although we're not fussy about use of imperative mood vs past tense. In particular, avoid commit messages that include [Angular-style metadata](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-header).
 
 * Please add test cases for your feature, and ensure all tests are passing (`npm run test`).