config time zone

Instruct user to set time zone option that was added in https://github.com/gitlabhq/gitlabhq/pull/8015
I think it should be something like
>If you were using a custom time zone in `config/...`, you can now set the time zone in `gitlab.yml` like this:...

So that if I am a user who never knew about custom timezones, I can read this paragraph, see I need to take no action, and immediately forget about it again.