Update Changelog

Add section in Changelog about unreleased version 0.15.1.

Changelog

@@ -6,6 +6,28 @@ Unified Changelog file for Synnefo versions >= 0.13
 Since v0.13 most of the Synnefo components have been merged into a single
 repository and have aligned versions.
 
+.. _Changelog-0.15.1:
+
+v0.15.1
+=======
+
+Released: UNRELEASED
+
+Astakos
+-------
+
+* Change statistics API to expose statistics for all users and for users
+  per authentication provider (users that are exclusively using a provider).
+
+Cyclades
+--------
+
+* Fix bug in snf-dispatcher that could result in unsynced NICs between
+  Cyclades and Ganeti.
+* Support queries by IPv6 address in helpdesk app.
+* Extend statistics API with detailed information about Ganeti clusters,
+  virtual servers and networks, public IPv4 pools and images.
+
 .. _Changelog-0.15:
 
 v0.15