Update link and instructions about ssh keys in HACKING

Gitweb changed their instructions, info about ssh keys is now on
separate page for all platforms (selector at top of the page).

Change-Id: I3eab5dfae06cfb73f4a76718f92518454021e557
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/768
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

HACKING

@@ -47,14 +47,8 @@ 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 for your specific platform:
+  -# Add an SSH public key following the directions on github:
-     - for Windows: http://help.github.com/win-set-up-git/#_set_up_ssh_keys
+     https://help.github.com/articles/generating-ssh-keys
-     - for OSX: http://help.github.com/mac-set-up-git/#_set_up_ssh_keys
-     - for Linux: http://help.github.com/linux-set-up-git/#_set_up_ssh_keys<br>
-     .
-     While these pages describe the setting up of git as well,
-     you should scroll down the page till you get to the section:
-     <i>Next: Set Up SSH Keys</i>, and follow the steps described.
 -# Clone the git repository, rather than just download the source:
  @code
  git clone git://openocd.git.sourceforge.net/gitroot/openocd/openocd