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>

HACKING

@@ -134,6 +134,10 @@ git config remote.review.push HEAD:refs/for/master
 @code
 git remote add review http://USERNAME:PASSWORD@openocd.zylin.com/p/openocd.git
 @endcode
+  \note All contributions should be pushed to @c refs/for/master on the
+Gerrit server, even if you plan to use several local branches for different
+topics. It is possible because @c for/master is not a traditional Git
+branch.
   -# You will need to install this hook, we will look into a better solution:
 @code
 scp -p -P 29418 USERNAME@openocd.zylin.com:hooks/commit-msg .git/hooks/