Adding a note to UPGRADE.txt about a change made to res_agi in order to

indicate when streaming an audio file fails like it is done in other parts
of the code to indicate an error.

Note was requested by Paul Belanger: 
http://lists.digium.com/pipermail/asterisk-dev/2013-July/061420.html

(related to issue ASTERISK-21903)
........

Merged revisions 396196 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@396197 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE.txt

@@ -19,6 +19,11 @@
 === UPGRADE-10.txt -- Upgrade info for 1.8 to 10
 ===
 ===========================================================
+from 11.5 to 11.6:
+* res_agi will now properly indicate if there was an error in streaming an
+  audio file.  The result code will be -1 and the result returned from the
+  the function will be RESULT_FAILURE instead of the prior behavior of always
+  returning RESULT_SUCCESS even if there was an error. 
 
 From 11.4 to 11.5:
 * The default settings for chan_sip are now overriden properly by the general