Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce

doc/system_hooks/system_hooks.md

doc/update/2.6-to-3.0.md

doc/update/4.2-to-5.0.md

doc/update/5.1-to-6.0.md

doc/update/6.1-to-6.2.md

doc/update/6.x-or-7.x-to-7.7.md

doc/update/7.3-to-7.4.md

doc/update/README.md

doc/workflow/gitlab_flow.md

doc/workflow/migrating_from_svn.md

doc/workflow/notifications.md