Added link to Image Preview cookbook entry from form_help_messages

Resources/doc/reference/form_help_message.rst

@@ -46,6 +46,15 @@ Alternative Way To Define Help Messages
         }
     }
 
+
+Advanced usage
+^^^^^^^^^^^^^^
+
+Since help messages can contain HTML they can be used for more advanced solutions.
+See the cookbook entry :doc:`Showing image previews <recipe_image_previews>` for a detailed example of how to 
+use help messages to display an image tag.
+
+
 Form Group Descriptions
 -----------------------
 
@@ -68,3 +77,4 @@ Example
                 ->end();
         }
     }
+