Add a link to how to set Omnibus environment variables

[ci skip]

doc/administration/environment_variables.md

@@ -59,6 +59,9 @@ to the naming scheme `GITLAB_#{name in 1_settings.rb in upper case}`.
 
 ## Omnibus configuration
 
+To set environment variables, follow [these
+instructions](https://docs.gitlab.com/omnibus/settings/environment-variables.html).
+
 It's possible to preconfigure the GitLab docker image by adding the environment
 variable `GITLAB_OMNIBUS_CONFIG` to the `docker run` command.
 For more information see the ['preconfigure-docker-container' section in the Omnibus documentation](http://docs.gitlab.com/omnibus/docker/#preconfigure-docker-container).