Document the git_data_dir attribute

README.md

@@ -98,6 +98,17 @@ Omnibus-gitlab uses four different directories.
 - `/var/log/gitlab` contains all log data generated by components of
   omnibus-gitlab.
+### Storing Git data in an alternative directory
+
+By default, omnibus-gitlab stores Git repository data in `/var/opt/gitlab/git-data`.
+You can change this location by adding the following line to `/etc/gitlab/gitlab.rb`.
+
+```ruby
+git_data_dir "/mnt/nas/gitlab"
+```
+
+Run `sudo gitlab-ctl reconfigure` for the change to take effect.
+
 ## Building your own package
 See [the separate build documentation](doc/build.md).