Added infos for ubuntu

git-svn-id: https://redmine.imt-atlantique.fr/svn/xaal/code/Python/trunk@2117 b32b6428-25c9-4566-ad07-03861ab6144f

README.rst

@@ -16,7 +16,7 @@ First build a virtualenv :
 
 .. code-block:: bash
                 
-   $ virtualenv3 xaal_env
+   $ virtualenv3 xaal_env (or python3 -m virtualenv --python=python3 xaal_env)
    $ source xaal_env/bin/activate
 
 Every time, you want to use the binding, you must source the activate script.