Update CHANGES file to include MWI subscription support that was added some time ago.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@177291 65c4cc65-6c06-0410-ace0-fbb531ad65f3

CHANGES

@@ -44,6 +44,8 @@ SIP Changes
  * The parsing of register => lines in sip.conf has been modified to allow a port
    to be present in the "user" portion. Please see the sip.conf.sample file for more
    information
+ * Added support for subscribing to MWI on a remote server and making the status available
+   as a mailbox. Please see the sip.conf.sample file for more information.
  * Added a function to remove SIP headers added in the dialplan before the
    first INVITE is generated - SIPRemoveHeader()
  * Channel variables set with setvar= in a device configuration is now