Add help text for wheelPropagation option.

README.md

@@ -79,8 +79,10 @@ The scroll speed applied to mousewheel event.
 
 ### wheelPropagation
 If this option is true, when the scroll reach the end of the side, mousewheel event will be propagated to parent element.  
+*Currently not supported for touch events*  
 **Default: false**
 
+
 ### minScrollbarLength
 When set to an integer value, the thumb part of the scrollbar will not shrink below that number of pixels.  
 **Default: null**