Clarify the mutation

Author: gaearon

examples/04 Sortable/Simple/Card.js

@@ -58,7 +58,10 @@ const cardTarget = {
     // Time to actually perform the action
     props.moveCard(dragIndex, hoverIndex);
 
-    // Update the monitors item with the new index
+    // Note: we're mutating the monitor item here!
+    // Generally it's better to avoid mutations,
+    // but it's good here for the sake of performance
+    // to avoid expensive index searches.
     monitor.getItem().index = hoverIndex;
   }
 };