Update stable branch example

The stable/juno branch was deleted 11 months ago:
http://lists.openstack.org/pipermail/openstack-announce/2015-December/000869.html

Update the example to Newton, which should keep it valid for a while.

Change-Id: I4cd8738862a529fd319be2ec5694d00defd94f84
diff --git a/README.md b/README.md
index 4ba4619..ff5598b 100644
--- a/README.md
+++ b/README.md
@@ -25,9 +25,9 @@
 The DevStack master branch generally points to trunk versions of OpenStack
 components.  For older, stable versions, look for branches named
 stable/[release] in the DevStack repo.  For example, you can do the
-following to create a juno OpenStack cloud:
+following to create a Newton OpenStack cloud:
 
-    git checkout stable/juno
+    git checkout stable/newton
     ./stack.sh
 
 You can also pick specific OpenStack project releases by setting the appropriate