Unhardcoding disk_cachemodes nova option

Change-Id: Ia327b4a13b40d3e8e136045190597bbb3c5a84cd
diff --git a/README.rst b/README.rst
index f7102e1..ac97a5c 100644
--- a/README.rst
+++ b/README.rst
@@ -157,6 +157,7 @@
         version: juno
         enabled: true
         virtualization: kvm
+        disk_cachemodes: network=writeback,block=none
         availability_zone: availability_zone_01
         aggregates:
         - hosts_with_fc