docs(quickstart): added install deisctl step at top of quick start docs

Subsequent quick start steps may not succeed if deisctl is not installed. E.g., the ./provision-aws-cluster.sh script stalled looking for a deisctl dependency. Added the deisctl install link to the "Get the Source" section, which is currently included inline at the top of the quick start docs.

Author: jackfrancis

docs/_includes/_get-the-source.rst

@@ -18,6 +18,8 @@ clone the source code into your `$GOPATH`_:
     $ go get -u -v github.com/deis/deis
     $ cd $GOPATH/src/github.com/deis/deis
 
+Additionally, you'll need the ``deisctl`` CLI tool. If you don't already have it,
+install instructions are :ref:`here <install_deisctl>`.
 
 .. _`source code`: https://github.com/deis/deis
 .. _`releases page`: https://github.com/deis/deis/releases

docs/installing_deis/quick-start.rst

@@ -64,7 +64,7 @@ Install Deis Platform
 ---------------------
 
 Now that you've finished provisioning a CoreOS cluster,
-please refer to :ref:`install_deisctl` and :ref:`install_deis_platform`.
+please :ref:`install_deis_platform`.
 
 
 .. _`CoreOS`: https://coreos.com/