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>

HACKING

@@ -47,8 +47,14 @@ Add yourself to the GPL copyright for non-trivial changes.
      add a username of your choice.
      Your username will be required in step 3 and substituted wherever
      the string 'USERNAME' is found.
-  -# Add an SSH public key following the directions on github:
-     https://help.github.com/articles/generating-ssh-keys
+  -# Create an SSH public key following the directions on github:
+     https://help.github.com/articles/generating-ssh-keys . You can skip step 3
+     (adding key to Github account) and 4 (testing) - these are useful only if
+     you actually use Github or want to test whether the new key works fine.
+  -# Add this new SSH key to your Gerrit account:
+     go to 'Settings' > 'SSH Public Keys', paste the contents of
+     ~/.ssh/id_rsa.pub into the text field (if it's not visible click on
+     'Add Key ...' button) and confirm by clicking 'Add' button.
 -# Clone the git repository, rather than just download the source:
  @code
  git clone git://git.code.sf.net/p/openocd/code openocd