Add a comment to gitlab.yml.example for cusom installing

Closed username-removed-16837 requested to merge team.hiroq/gitlab-ce:master into master

I've setup production server on my VM following installation.

When I tried to use "relative_url_root", I faced some error and needed to search information about it.

So I added a maybe helpful comment to gitlab.yml.example to avoid above error.

Finally, thank you for hosting such a great project!