Add missing news item about xlim/ylim

Author: hadley

NEWS

@@ -20,4 +20,5 @@ ggplot2 0.8.5 (2009-XX-XX) ----------------------------------------
   (Implements #27)
 * aesthetics: when _setting_ an aesthetic, you may only set it to a single 
   value.  This was always a good idea, but now it is enforced with an 
-  informative error message.
+  informative error message.
+* xlim, ylim: fix bug when setting limits of discrete scales