README.md: avoid Github pull requests

Signed-off-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Reviewed-by: Ganesh Ajjanagadde <gajjanag@mit.edu>

README.md

@@ -40,3 +40,10 @@ Coding examples are available in the **doc/examples** directory.
 
 FFmpeg codebase is mainly LGPL-licensed with optional components licensed under
 GPL. Please refer to the LICENSE file for detailed information.
+
+## Contributing
+
+Patches should be submitted to the ffmpeg-devel mailing list using
+`git format-patch` or `git send-email`. Github pull requests should be
+avoided because they are not part of our review process. Few developers
+follow pull requests so they will likely be ignored.