diff --git a/README.txt b/README.txt
index fe044a4..761c47c 100644
--- a/README.txt
+++ b/README.txt
@@ -36,7 +36,8 @@ http://example.com/files/imagecache/product_thumbnail/files/green-widget...
 
 NOTE: Each role that wishes to view the images generated by a
 particular preset must be given permission on the admin/user/permissions
-page.
+page. This only works for private file systems, and these permissions will
+be ignored on a public file system.
 
 ImageCache also defines a theme function that you can use in your modules and
 themes to automatically display a manipulated image. For example, to use the