Add tree/dag example

Author: vasturiano

README.md

@@ -31,6 +31,7 @@ Check out the examples:
 * [Click to focus on node](https://vasturiano.github.io/react-force-graph/example/click-to-focus/) ([source](https://github.com/vasturiano/react-force-graph/blob/master/example/click-to-focus/index.html))
 * [Camera automatic orbitting](https://vasturiano.github.io/react-force-graph/example/camera-auto-orbit/) ([source](https://github.com/vasturiano/react-force-graph/blob/master/example/camera-auto-orbit/index.html))
 * [Node collision detection](https://vasturiano.github.io/react-force-graph/example/collision-detection/) ([source](https://github.com/vasturiano/react-force-graph/blob/master/example/collision-detection/index.html))
+* [Force-directed tree (DAG mode)](https://vasturiano.github.io/react-force-graph/example/tree/) ([source](https://github.com/vasturiano/react-force-graph/blob/master/example/tree/index.html))
 
 ## Quick start