update order of upgrade guide

GitLab needs to be stopped when backup is took.
Add documentation for the Sidekiq MemoryKiller
Remove unfinished file with the wrong name
Add link to 'operations' README
Update changelog
Merge branch 'update_changelog_7_6' into 'master'

Update changelog 7.6

See merge request !1327
Merge branch 'doc/sidekiq_memory_killer' into 'master'

Add documentation for the Sidekiq MemoryKiller

We already merged the code, and we are already using it on gitlab.com. This MR adds documentation.

See merge request !1325
add kerberos to Gemfile
Merge branch 'update_gemfile_with_kerberos' into 'master'

Add kerberos to gemfile

See merge request !1330
Move development information to the GitLab Development Kit.
Merge pull request #8375 from bbodenmiller/process-fix

update order of upgrade guide
Merge branch 'dev-docs-in-gdk' into 'master'

Dev docs in gdk

Also see https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/31

See merge request !1331
Merge branch 'master' of github.com:gitlabhq/gitlabhq
Include default credentials in the readme and make all headers the same.
Merge branch 'credentials-in-readme' into 'master'

Credentials in readme

See merge request !1332
Merge branch 'master' of dev.gitlab.org:gitlab/gitlabhq
Added update guide for updating to 7.6
Fixed version reference
Merge branch 'update-doc-7-6' into 'master'

Update Documentation for 7.6 release

Updated the update and install documentation for the 7.6 release.

See merge request !1334
Version 7.6.0.rc1