[libpng16] Began a manual section about libpng15 to libpng16 differences.
diff --git a/png.h b/png.h
index b85e6a6..2c7bb3e 100644
--- a/png.h
+++ b/png.h
@@ -2596,8 +2596,8 @@
  *  SIMPLIFIED API
  *******************************************************************************
  *
- * Please read the documentation in libpng-manual.txt if you don't understand
- * what follows.
+ * Please read the documentation in libpng-manual.txt (TODO: write said
+ * documentation) if you don't understand what follows.
  *
  * The simplified API hides the details of both libpng and the PNG file format
  * itself.  It allows PNG files to be read into a very limited number of