HACKING: add note about refs/for/master

HACKING: add note about refs/for/master

The fact that one needs to always push contributions to a single
remote reference (refs/for/master) might seem odd to people unfamiliar
with Gerrit. GitHub, for instance, hosts personal repositories where
developers typically create topic branches for each contribution and
use a proprietary mecanism to request a review (the "pull request").
More generally, one normally does not expect to be able to push
non-fast-forwarding stuff to a remote branch.

This commit adds a clarifying note to the patch guidelines.

Change-Id: Ia750b815b82b18e92b6109c07f451000dcbecf9b
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-on: http://openocd.zylin.com/4806
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>