Add ref to Installation from Usage doc.

Author: mboersma

docs/gettingstarted/usage.rst

@@ -7,8 +7,8 @@
 Usage
 =====
 
-Once you have set up a Deis Controller, you will use a CLI 
-(command-line interface) to create a formation for your app, 
-push your code, and scale your own cloud resources.
+Once you have set up a Deis Controller (see :ref:`installation`), you
+will use the Deis CLI (command-line interface) to create a formation for
+your app, push your code, and scale your own cloud resources.
 
 .. automodule:: client.deis