Adds new section for changing time zone in GitLab configuration file. New...

Adds new section for changing time zone in GitLab configuration file. New section added for changing time zone in Omnibus installations.

doc/workflow/timezone.md

 # Changing your time zone
 
-GitLab defaults its time zone to UTC. It has a global timezone configuration parameter in /etc/gitlab/gitlab.rb 
+The global time zone configuration parameter can be changed in `config/gitlab.yml`:
+```
+    # time_zone: 'UTC'
+```
+
+Uncomment and customize if you want to change the default time zone of GitLab application.
+
+To see all available time zones, run `bundle exec rake time:zones:all`.
+
+
+## Changing time zone in omnibus installations
+
+GitLab defaults its time zone to UTC. It has a global timezone configuration parameter in `/etc/gitlab/gitlab.rb`.
 
 To update, add the time zone that best applies to your location. Here are two examples:
 ```