Improve the CHANGELOG entry [ci skip]

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
-*   `composed_of` has removed. You'll have to write your own accessor
+*   `composed_of` was removed. You'll have to write your own accessor
     and mutator methods if you'd like to use value objects to represent some
-    portion of your models.
+    portion of your models. So, instead of:
+
+        class Person < ActiveRecord::Base
+          composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
+        end
+
+    you could write something like this:
+
+        def address
+          @address ||= Address.new(address_street, address_city)
+        end
+
+        def address=(address)
+          self[:address_street] = @address.street
+          self[:address_city]   = @address.city
+
+          @address = address
+        end
 
     *Steve Klabnik*