CONTRIBUTING.md: clarify AI usage in contribution trailers

Signed-off-by: Wei Fu <fuweid89@gmail.com>

Author: fuweid

CONTRIBUTING.md

@@ -148,5 +148,10 @@ Then you just add a line to every git commit message:
 
 Use your real name (sorry, no pseudonyms or anonymous contributions.)
 
+AI tools, services, or assistants may be used to assist with contributions, but
+do not add them to `Signed-off-by` or `Co-authored-by` lines. These trailers
+must identify the human contributors certifying or authoring the contribution,
+or project-approved automation accounts used to submit automated contributions.
+
 If you set your `user.name` and `user.email` git configs, you can sign your
 commit automatically with `git commit -s`.