Add canDrag documentation

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>
+canDrag                   | func or bool   | `true`              |          | Return false from callback to prevent node from dragging, by hiding the drag handle. Set prop to `false` to disable dragging on all nodes. <div>`({ node: object, path: number[] or string[], treeIndex: number, lowerSiblingCounts: number[], isSearchMatch: bool, isSearchFocus: bool }): bool`</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`

src/examples/basicExample/app.js

@@ -124,6 +124,11 @@ class App extends Component {
                                 },
                             ],
                         },
+                        {
+                            title: 'Disable dragging on a per-node basis with the `canDrag` prop',
+                            subtitle: 'Or set it to false to disable all dragging.',
+                            noDragging: true,
+                        },
                         {
                             title: 'You cannot give this children',
                             subtitle: 'Dropping is prevented via the `canDrop` API using `nextParent`',
@@ -287,6 +292,7 @@ class App extends Component {
                             maxDepth={maxDepth}
                             searchQuery={searchString}
                             searchFocusOffset={searchFocusIndex}
+                            canDrag={({ node }) => !node.noDragging}
                             canDrop={({ nextParent }) => !nextParent || !nextParent.noChildren}
                             searchFinishCallback={matches =>
                                 this.setState({