How to install drush extension / addon globally?
Put it in /usr/share/drush/commands for global accessibility.  On Windows, Drush will look in the folder named Drush/share/drush/commands inside the location specified by the ALLUSERSPROFILE environment variable.

After adding new commands, you will need to run drush cc drush to clear the Drush command cache, so that Drush will once again scan its command searchpath.