ExternalIVR information for UPGRADE.txt

added a paragraph about the fixes and changes to
the ExternalIVR application.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@240974 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE.txt

@@ -61,6 +61,12 @@ From 1.6.2 to 1.8:
   is written to expect AVAILSTATUS to contain the cause code it needs to be
   changed to use AVAILCAUSECODE.
 
+* ExternalIVR will now send Z events for invalid or missing files, T events
+  now include the interrupted file and bugs in argument parsing have been
+  fixed so there may be arguments specified in incorrect ways that were
+  working that will no longer work.
+  Please see doc/externalivr.txt for details.
+
 From 1.6.1 to 1.6.2:
 
 * SIP no longer sends the 183 progress message for early media by