Update CONTRIBUTING.md

Author: bep

CONTRIBUTING.md

@@ -19,6 +19,7 @@ The Hugo community and maintainers are [very active](https://github.com/gohugoio
 * [Reporting Issues](#reporting-issues)
 * [Submitting Patches](#submitting-patches)
   * [Code Contribution Guidelines](#code-contribution-guidelines)
+  * [AI Assistance Notice](#ai-assistance-notice)
   * [Git Commit Message Guidelines](#git-commit-message-guidelines)
   * [Fetching the Sources From GitHub](#fetching-the-sources-from-github)
   * [Building Hugo with Your Changes](#building-hugo-with-your-changes)
@@ -79,6 +80,16 @@ To make the contribution process as seamless as possible, we ask for the followi
     * Ensure that `mage check` succeeds. [Travis CI](https://travis-ci.org/gohugoio/hugo) (Windows, Linux and macOS) will fail the build if `mage check` fails.
     * Follow the **Git Commit Message Guidelines** below.
 
+## AI Assistance Notice
+
+If a substantial part of your contribution is autogenerated with AI, **this must be disclosed in the pull request**, along with the extent to which AI assistance was used.
+
+An example disclosure:
+
+> This PR was written primarily by Claude Code.
+
+When using AI assistance, we expect contributors to understand the code that is produced and be able to answer critical questions about it.
+
 ### Git Commit Message Guidelines
 
 This [blog article](https://cbea.ms/git-commit/) is a good resource for learning how to write good commit messages,