Add note about the out-of-tree module ABI changes.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@254098 65c4cc65-6c06-0410-ace0-fbb531ad65f3

UPGRADE.txt

@@ -501,6 +501,13 @@ API:
   res_agi, you will need to add ast_ prefixes to the functions that you call
   from those modules.
 
+ABI:
+
+* Many locking-related functions have had their ABI changed.  This may cause
+  external modules (including those provided by Asterisk-addons) to fail to
+  load with 'symbol not found' errors.  These modules will need to be
+  recompiled against a more current version of Asterisk.
+
 Formats:
 
 * format_wav: The GAIN preprocessor definition has been changed from 2 to 0