Add create stack user to quickstart document

Add instructions on creating a user to the documentation, and call out
that the standard cloud-users are probably ok too

Change-Id: I1119a43f1d5ae7c0c208bf0cc16e2f7bee29a69d
diff --git a/doc/source/index.rst b/doc/source/index.rst
index b65730f..ec345c9 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -44,6 +44,18 @@
 
    We recommend at least a :ref:`minimal-configuration` be set up.
 
+#. Add Stack User
+
+   Devstack should be run as a non-root user with sudo enabled
+   (standard logins to cloud images such as "ubuntu" or "cloud-user"
+   are usually fine).
+
+   You can quickly create a separate `stack` user to run DevStack with
+
+   ::
+
+       devstack/tools/create-stack-user.sh; su stack
+
 #. Start the install
 
    ::