Update README with canDrop info

Author: fritz-c

README.md

@@ -58,6 +58,7 @@ generateNodeProps         | func           |                     |          | Ge
 getNodeKey                | func           | defaultGetNodeKey   |          | Determine the unique key used to identify each node and generate the `path` array passed in callbacks. By default, returns the index in the tree (omitting hidden nodes).<div>`({ node: object, treeIndex: number }): string or number`</div>
 onMoveNode                | func           |                     |          | Called after node move operation. <div>`({ treeData: object[], node: object, treeIndex: number, path: number[] or string[] }): void`</div>
 onVisibilityToggle        | func           |                     |          | Called after children nodes collapsed or expanded. <div>`({ treeData: object[], node: object, expanded: bool }): void`</div>
+canDrop                   | func           |                     |          | Return false to prevent node from dropping in the given location. <div>`({ node: object, prevPath: number[] or string[], prevParent: object, nextPath: number[] or string[], nextParent: object}): bool`</div>
 reactVirtualizedListProps | object         |                     |          | Custom properties to hand to the [react-virtualized list](https://github.com/bvaughn/react-virtualized/blob/master/docs/List.md#prop-types)
 rowHeight                 | number or func | `62`                |          | Used by react-virtualized. Either a fixed row height (number) or a function that returns the height of a row given its index: `({ index: number }): number`
 slideRegionSize           | number         | `100`               |          | Size in px of the region near the edges that initiates scrolling on dragover.