added UPDATE notes for Serializer component changes

UPDATE.md

@@ -9,6 +9,10 @@ timeline closely anyway.
 beta4 to beta5
 --------------
 
+* Expanded the SerializerInterface, while reducing the number of public
+  methods in the Serializer class itself and adding component specific
+  Exception classes.
+
 * The temporary storage for file uploads has been removed
 
 * The `Symfony\Component\HttpFoundation\File\File::getExtension()` and