Add a screenshot in README

As there is no public demo of coin, this is the least we can show.

README.md

@@ -4,6 +4,9 @@ The COIN project
 `Coin` is Illyse's Information System, designed to manage both members
 and Internet accesses, such as through DSL, VPN, wireless…
 
+
+![example of a coin user view : the subscriptions list](doc/_img/user-subscriptions.png)
+
 It is written in Django, and features a generic configuration interface,
 which allows to implement custom backends for different technologies.
 Currently implemented is a LDAP-based backend for OpenVPN, and a very