Move custom Omniauth directions to omniauth doc.
Link the doc in installation document.
Merge branch 'omniauth_doc' into 'master'

Omniauth doc

Fixes #1481

See merge request !1069
Fix images on GitHub and link to the about url.
Merge branch 'fix-images-on-github' into 'master'

Fix images on github

Fixes https://dev.gitlab.org/gitlab/organization/issues/55

/cc @job @dzaporozhets

See merge request !1073
Merge branch 'master' of github.com:gitlabhq/gitlabhq