add change note

CHANGES.txt

@@ -10,6 +10,11 @@
   opened than permitted by the environment when starting a bunch of programs.
   Now, instead a spawn error is logged.
 
+- Compute "channel delay" properly, fixing symptoms where a supervisorctl start
+  command would hang for a very long time when a process (or many processes)
+  are spewing to their stdout or stderr.  See comments attached to
+  https://github.com/Supervisor/supervisor/pull/263 .
+
 3.1.0 (2014-07-29)
 ------------------