Adding ability to set up optional groups for user.

Change-Id: I0109e9b5298253fac0dac090066c313e8f7fccbb
diff --git a/README.rst b/README.rst
index 79b8b18..0db113f 100644
--- a/README.rst
+++ b/README.rst
@@ -51,6 +51,11 @@
             home_dir_mode: 755
             email: 'jonh@doe.com'
             unique: false
+            groups:
+            - db-ops
+            - salt-ops
+            optional_groups:
+            - docker
           jsmith:
             name: 'jsmith'
             enabled: true