add reference to bug report for PNG depth handling.
diff --git a/ChangeLog b/ChangeLog
index 4f51a26..43813a8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,7 @@
2011-05-20 6.6.9-10 Glenn Randers-Pehrson <glennrp@image...>
* The PNG encoder now respects the -depth setting, including 1, 2, 4.
- Previously settings other than 8 and 16 were ignored.
+ Previously settings other than 8 and 16 were ignored (reference
+ http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=18412).
2011-05-16 6.6.9-10 Cristy <quetzlzacatenango@image...>
* Don't use comma as a separator for stroked text (reference