New documentation for updating patches for gerrit

Added instructions on how to amend changes to a previous commit.
Included commands and brief instructions.

Change-Id: I078c9392d71273f559357a014768aada85ad3405
Signed-off-by: Kyle Schneider <kinnder@gmail.com>
Reviewed-on: http://review.gluster.org/5295
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>