Update HACKING info

Inform about possibility of discarding negative review in Gerrit when specific conditions are met.

Change-Id: I432b6c93cefc368fa22ce1096bea4cd174e03816
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/747
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Tested-by: Øyvind Harboe <oyvindharboe@gmail.com>