Discouraged filing PRs that won't be noticed.

Author: aaugustin

CONTRIBUTING.rst

@@ -14,3 +14,12 @@ Extensive contribution guidelines are available in the repository at
 ``docs/internals/contributing/``, or online at:
 
 https://docs.djangoproject.com/en/dev/internals/contributing/
+
+**Warning: pull requests are ignored!** `File a ticket`__ to suggest changes.
+
+__ https://code.djangoproject.com/newticket
+
+Django uses Trac to keep track of bugs, feature requests, and associated
+patches because GitHub doesn't provide adequate tooling for its community.
+Patches can be submitted as pull requests, but if you don't file a ticket,
+it's unlikely that we'll notice your contribution.