Explain messages with patches in CONTRIBUTE

* CONTRIBUTE: A message shall contain the string "[PATCH]" in the
subject if there's a patch in the message.

CONTRIBUTE

@@ -59,7 +59,10 @@ such patch without additional remarks, you can use a command like
 The Emacs issue tracker at https://debbugs.gnu.org lets you view bug
 reports and search the database for bugs matching several criteria.
 Messages posted to the bug-gnu-emacs@gnu.org mailing list, mentioned
-above, are recorded by the tracker with the corresponding bugs/issues.
+above, are recorded by the tracker with the corresponding
+bugs/issues.  If a message to the bug tracker contains a patch, please
+include the string "[PATCH]" in the subject of the message in order to
+let the bug tracker tag the bug properly.
 
 GNU ELPA has a 'debbugs' package that allows accessing the tracker
 database from Emacs.