Add possibility to specify Compose file version

This change allows to set the Compose file version in the model instead
of the hardcoded value.

Change-Id: I5f7fb3c0770a48fe6dada54ffb9f925ef6ebd078
diff --git a/README.rst b/README.rst
index deeb54d..ef6b711 100644
--- a/README.rst
+++ b/README.rst
@@ -119,6 +119,7 @@
             update: true
             environment:
               SOMEVAR: somevalue
+            version: "3.1"
             service:
               db:
                 image: postgres