Add feature gaps for Pivotal Tile

doc/install/pivotal/index.md

@@ -19,6 +19,19 @@ and click a button to begin the upgrade process.
 - You want a highly available deployment with minimal effort. Scale horizontally
   as your user base grows.
 
+## Features
+
+The GitLab Pivotal Tile is based on [Enterprise Edition Premium] and includes nearly all of its features. The features in EEP but _not_ supported on the Tile are:
+
+* PostgreSQL
+* Pages
+* Geo
+* Mattermost
+* Subgroups
+* Elasticsearch
+* Service Desk
+* OAuth & Kerberos Authentication
+
 ## Installing GitLab with Pivotal
 
 The product information and installation documentation is hosted on Pivotal's