fix(docs): mention that Deis requires 3 or more machines

Author: mboersma

docs/installing_deis/index.rst

@@ -1,19 +1,18 @@
 :title: Installing Deis
-:description: Step-by-step guide for operations engineers setting up a private PaaS using Deis.
+:description: Step-by-step guide for operations engineers setting up Deis.
 
 .. _installing_deis:
-.. _provision-controller:
 
 Installing Deis
 ===============
 
-Provisioning Deis is a matter of creating one or more :ref:`concepts_coreos`
-machines and using :ref:`install_deisctl` to install and start Deis.
+To install Deis consists of provisioning three or more :ref:`concepts_coreos`
+machines and using :ref:`deisctl <install_deisctl>` to set up and start
+the software.
 
-Anywhere you can run CoreOS, you can run Deis, including most cloud
-providers, virtual machines, and bare metal. See the
-`CoreOS documentation`_ for more information on how to get set up
-with CoreOS.
+Anywhere you can run CoreOS, you can run Deis, including most cloud providers,
+virtual machines, and bare metal. See the `CoreOS documentation`_ for more
+information on how to get set up with CoreOS.
 
 :Release: |version|
 :Date: |today|