Include CHANGES.txt note for issue2550870

Author: jerrykan

CHANGES.txt

@@ -149,6 +149,8 @@ Fixed:
 - issue2550877 Failures in test_mailgw.py because of duplicated headers
   and more precise comparision. Writing headers with the email module will use 
   continuation_ws = ' ' now for python 2.5 and 2.6. (Bernhard Reiter)
+- issue2550870 migrate use of 'rfc822' module to the 'email' module
+  (Bernhard Reiter/John Kristensen)
 
 
 2013-07-06: 1.5.0