added upgrading file

UPGRADING.md

@@ -0,0 +1,16 @@
+This document details the changes that you need to make to your code
+when upgrading from one version to another.
+
+Upgrading From 0.9 to 1.0
+=========================
+
+- Custom Handlers
+
+    The interfaces ``SerializationHandlerInterface``, and ``DeserializationHandlerInterface``
+    have been removed. Instead, you can now use either an event listener, or the new handler
+    concept. As a general rule, if your handler was registered for a specific type, you
+    would use the new handler system, if you instead were handling an arbitrary number of
+    possibly unknown types, you would use the event system.
+
+    Please see the documentation for how to set-uup one of these.
+