add subscriber info to EE repo

doc/Subscriber_information.md

+# Subscriber information
+
+Thank you for purchasing a GitLab subscription!
+
+If you would like to receive access to GitLab Enterprise Edition please create an account on https://gitlab.com/users/sign_up and send us the username. Note that your user will be visible to other Support Subscribers.
+
+Once your username is added, you can find the repository here:
+https://gitlab.com/subscribers/gitlab-ee
+
+Packages:
+https://gitlab.com/subscribers/gitlab-ee/blob/master/doc/install/packages.md
+
+Documentation:
+http://doc.gitlab.com/ee/
+
+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
+
+If you need help with your GitLab installation and for any technical questions please contact us at subscribers@gitlab.com.
+
+For all other questions, contact us at sales@gitlab.com