Adjust contributing guidelines to help deal with AI

Author: Byron

CONTRIBUTING.md

@@ -1,3 +1,7 @@
-For now, please have a look at the section in the [README] file.
+### Please disclose the use AI…
+
+…if it's *full agent mode* or *multi*-line completions in the PR comment.
+
+For everything else, please have a look at the respective section in the [README] file.
 
 [README]: https://github.com/GitoxideLabs/gitoxide#contributions