Using sudo in the Quick start section

Adding sudo to the example commands in the quick start section.

Also adding '-' as su argument in order to use
the stack user's env (home).

Change-Id: I23ab38104d05c3f4c8d48b55e66cf19dc4e4f90d
diff --git a/doc/source/index.rst b/doc/source/index.rst
index b8dd506..edd6595 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -56,15 +56,15 @@
 
 ::
 
-   $ adduser stack
+   $ sudo adduser stack
 
 Since this user will be making many changes to your system, it should
 have sudo privileges:
 
 ::
 
-    $ echo "stack ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
-    $ su stack
+    $ sudo tee <<<"stack ALL=(ALL) NOPASSWD: ALL" /etc/sudoers
+    $ sudo su - stack
 
 Download DevStack
 -----------------