Update CI README to clarify setting the GitLab URL
Update CI README to clarify setting the GitLab URL

It's a (semi) common problem to point the CI server back at itself
rather than pointing at the GitLab server. See:
https://forum.gitlab.com/t/solved-404-oauth-no-route-matches-get-oauth-authorize/213/3

felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,felixbuenemann/omnibus-gitlab,felixbuenemann/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,copystudy/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,copystudy/omnibus-gitlab