Merge branch 'add_doc_to_upgrade_ce_to_ee' into '7-4-stable-ee'

Add reference to the README  to upgrade guide CE to EE

Issue - https://dev.gitlab.org/gitlab/gitlab-ee/issues/183

See merge request !213

README.md

 # ![logo](https://about.gitlab.com/images/gitlab_logo.png) GitLab
 
+For upgrading from GitLab CE to GitLab EE there is [upgrade guide](doc/update/7.4-ce-to-ee.md)
+
 ## Open source software to collaborate on code
 
 ![Animated screenshots](https://about.gitlab.com/images/animated/compiled.gif)