README.rst: Note GitHub PRs are auto-closed

This makes it a little bit clearer that GitHub pull requests are not
accepted and will be automatically closed.

This is a small tweak to help with one of the issues mentioned here:

http://www.alexconrad.org/2014/04/the-painful-process-of-submitting-your.html

Change-Id: Ie4aa8297a75f00ddd9e6c0fce722aa7a6586d52d