Added documentation for Geo

doc/gitlab-geo/README.md

+## GitLab Geo (EE)
+
+GitLab Geo allows you to replicate your GitLab instance to other
+geographical locations as a read-only fully operational version.
+
+When Geo is enabled, we reffer to your original instance as a **primary**
+node and the replicated read-only ones as **secondaries**.
+
+GitLab Geo requires some additional work installing and configuring your
+instance, than a normal setup.
+
+### Primary Node
+
+To make your day-use instance a primary Geo node, you access your
+administration screen and go to Gitlab Geo `/admin/geo_nodes`.
+
+You must add your instance address the same way it is configured on
+your `gitlab.yml`, select **this is a primary node** and save.
+
+### Secondary Node
+
+To install a secondary node, you must follow your normal install
+instructions with some extra requirements:
+ 
+ * You must replicate your database to this instance.
+ * Primary node must be able to access this instance by HTTP/HTTPS
+
+### Current limitations
+
+ * You cannot push code to secondary nodes
+ * Git LFS is not supported yet
+ * Git Annex is not supported yet