Merge branch 'docs/hangouts-introduced' into 'master'

Add when the hangouts integration was introduced

See merge request gitlab-org/gitlab-ce!21257

doc/user/project/integrations/hangouts_chat.md

 # Hangouts Chat service
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/43756) in GitLab 11.2.
+
 The Hangouts Chat service sends notifications from GitLab to the room for which the webhook was created.
 
 ## On Hangouts Chat