update README

Author: attilaolah

README.md

@@ -66,7 +66,11 @@ var wow = new WOW(
     animateClass: 'animated', // animation css class (default is animated)
     offset:       0,          // distance to the element when triggering the animation (default is 0)
     mobile:       true,       // trigger animations on mobile devices (default is true)
-    live:         true        // act on asynchronously loaded content (default is true)
+    live:         true,       // act on asynchronously loaded content (default is true)
+    callback:     function(box) {
+      // the callback is fired every time an animation is started
+      // the argument that is passed in is the DOM node being animated
+    }
   }
 );
 wow.init();