Mention how to enable LDAP integration

doc/integration/ldap.md

 # GitLab LDAP integration
 
 GitLab can be configured to allow your users to sign with their LDAP credentials to integrate with e.g. Active Directory.
+To enable LDAP integration, edit [gitlab.rb (omnibus-gitlab)`](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/README.md#setting-up-ldap-sign-in) or [gitlab.yml (source installations)](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/gitlab.yml.example) on your GitLab server and restart GitLab.
 
 The first time a user signs in with LDAP credentials, GitLab will create a new GitLab user associated with the LDAP Distinguished Name (DN) of the LDAP user.