Update README.md: info about custom autoscrolling function

README.md

@@ -74,6 +74,7 @@ var sortable = new Sortable(el, {
 	fallbackTolerance: 0 // Specify in pixels how far the mouse should move before it's considered as a drag.        
 	
 	scroll: true, // or HTMLElement
+	scrollFn: function(offsetX, offsetY, originalEvent) { ... }, // if you have custom scrollbar scrollFn may be used for autoscrolling
 	scrollSensitivity: 30, // px, how near the mouse must be to an edge to start scrolling.
 	scrollSpeed: 10, // px
 	
@@ -336,6 +337,14 @@ Demo:
 ---
 
 
+#### `scrollFn` option
+Defines function that will be used for autoscrolling. el.scrollTop/el.scrollLeft is used by default.
+Useful when you have custom scrollbar with dedicated scroll function.
+
+
+---
+
+
 #### `scrollSensitivity` option
 Defines how near the mouse must be to an edge to start scrolling.