Improve HACKING

Reword info about creating SSH key - it's not required to add it to
Github account. Mention adding created SSH key to Gerrit account -
without this step it's not possible to access Gerrit in further
steps.

Change-Id: Ibd81521fbe47d4b4beae0b77cdc9d939fd3ee20c
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/1350
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>