Add a combined update doc from 5.1 to 6.0
Report application status through init exit code
Refer to disclosure policy in CONTRIBUTING.md
Merge branch 'init_exit_status' of /home/git/repositories/gitlab/gitlabhq
Merge branch 'security_vulnerabilities' of /home/git/repositories/gitlab/gitlabhq
Merge branch '51-to-60-update' of /home/git/repositories/gitlab/gitlabhq