verify-commits: Mention git v2.38.0 requirement

Author: achow101

contrib/verify-commits/README.md

@@ -27,6 +27,10 @@ Note that the above isn't a good UI/UX yet, and needs significant improvements
 to make it more convenient and reduce the chance of errors; pull-reqs
 improving this process would be much appreciated.
 
+Unless `--clean-merge 0` is specified, `verify-commits.py` will attempt to verify that
+each merge commit applies cleanly (with some exceptions). This requires using at least
+git v2.38.0.
+
 Configuration files
 -------------------