[devel] Note in CHANGES that pngusr.h is unuseable

and fixed a small typo in a comment in pngconf.h
diff --git a/CHANGES b/CHANGES
index 8c75be8..dae7779 100644
--- a/CHANGES
+++ b/CHANGES
@@ -2642,6 +2642,7 @@
     in the new installed header file pnglibconf.h
   Removed the xcode project because it has not been updated to work
     with libpng-1.5.0.
+  Removed the ability to include optional pngusr.h
 
 version 1.5.0beta18 [April 17, 2010]