Merge branch 'mrchrisw/oauth-callback' into 'master'

Include default callback URL (OAuth)

## What does this MR do?
Adds information on the OAuth callback URL

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
This information was missing.

## What are the relevant issue numbers?
http://www.twitter.com/designingcode/status/753511397124866049

## Does this MR meet the acceptance criteria?

- [X ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)

See merge request !5246

doc/integration/oauth_provider.md

@@ -28,7 +28,8 @@ GitLab supports two ways of adding a new OAuth2 application to an instance. You
 can either add an application as a regular user or add it in the admin area.
 What this means is that GitLab can actually have instance-wide and a user-wide
 applications. There is no difference between them except for the different
-permission levels they are set (user/admin).
+permission levels they are set (user/admin). The default callback URL is 
+`http://your-gitlab.example.com/users/auth/gitlab/callback`
 
 ## Adding an application through the profile