etc/TODO: add note about using undo-tree for persistent undo

etc/ChangeLog

+2014-05-28  Reuben Thomas  <rrt@sc3d.org>
+
+	* TODO: add a note that undo-tree could be used to save undo
+	information (Bug #17581).
+
 2014-05-27  Fabrice Popineau  <fabrice.popineau@gmail.com>
 
 	* NEWS: Mention build changes on MS-Windows.

etc/TODO

@@ -428,6 +428,8 @@ rather than interactively.  This a trivial one-liner in easy-mode.el.
 
 ** Save undo information in special temporary files, and reload it
   when needed for undoing.  This could extend undo capacity.
+  undo-tree, in ELPA, already does this; its saving code could be
+  integrated without requiring the use of undo-tree.
 
 ** Change the Windows NT menu code
   so that it handles the deep_p argument and avoids