Add libravatar documentation.
Add a note about notification for a project.
Different wording for notification note.
Add a link to libravatar doc in gitlab.yml.example.
Another link to GitHub flow.
Merge branch 'notification_level_note' into 'master'

Notification level note

Fixes #1406

![Screenshot 2014-10-14 12.28.40](https://dev.gitlab.org/uploads/gitlab/gitlabhq/e2aef0a8d6/Screenshot_2014-10-14_12.28.40.png)

See merge request !1174
Merge branch 'libravatar_documentation' into 'master'

Libravatar documentation

Fixes #1248

See merge request !1173