Merge branch 'slack-doc-readme' into 'master'

Slack doc readme

doc/integration/README.md

@@ -6,3 +6,4 @@ See the documentation below for details on how to configure these services.
 + [External issue tracker](external-issue-tracker.md) Redmine, JIRA, etc.
 + [LDAP](ldap.md) Set up sign in via LDAP
 + [OmniAuth](omniauth.md) Sign in via Twitter, GitHub, and Google via OAuth.
++ [Slack](slack.md) Integrate with the Slack chat service