Add link to services api doc.

Just a one-line change to link in the services api doc.

I'm curious though. Only gitlab-ci and hipchat are supported.  Would you accept a pull request for the other services? Actually I [submitted](http://feedback.gitlab.com/forums/176466-general/suggestions/7667256-add-api-calls-for-all-services-in-a-project) that last question to the feedback tool.

See merge request !544