[ADD] example in description

auth_dynamic_groups/__openerp__.py

@@ -39,6 +39,18 @@ truthy if the user is supposed to be a member of the group and falsy if not.
 There is a constraint on the field to check for validity if this expression.
 When you're satisfied, click the button `Evaluate` to prefill the group's
 members. The condition will be checked now for every user who logs in.
+
+Example
+-------
+We have a group called `Amsterdam` and want it to contain all users from
+city of Amsterdam. So we use the membership condition
+
+```
+user.partner_id.city == 'Amsterdam'
+```
+
+Now we can be sure every user living in this city is in the right group, and we
+can start assigning local menus to it, adjust permissions, etc.
     """,
     "category": "Tools",
     "depends": [