add a simple backport policy to README.GIT

This will clarify where patches should go, and who can do backports.

Reviewed-on: http://gerrit.openafs.org/8075
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 8662592caedbcf8e065312fe142731faabe1eb77)

Change-Id: I1d2c452dd17c8ae63eebfd8d800659c9de0fc327
Reviewed-on: http://gerrit.openafs.org/9684
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>