Add EE to update instructions https://about.gitlab.com/update

This is what @Haydn now sends:

In this case, to upgrade from CE, just perform a normal upgrade, but make use of an EE package: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/update.md#updating-from-gitlab-6-6-x-and-higher-to-the-latest-version

Here is the documentation on setting up the Jira and Jenkins integrations: http://doc.gitlab.com/ee/integration/jira.html http://doc.gitlab.com/ee/integration/jenkins.html

Can you do this @JobV ?