Update README

Author: Zachary Marion

README.md

@@ -2,16 +2,26 @@
 
 ![React svg tree](https://raw.githubusercontent.com/zacharyfmarion/react-svg-tree/master/assets/tree.png)
 
-React Tree provides components for rendering tree graphs with svg. It uses the algorithm described in [this paper](http://www.cs.unc.edu/techreports/89-034.pdf). You can pass in a map of the vertices to their children:
+React Tree provides components for rendering tree graphs with svg. It uses the algorithm described in [this paper](http://www.cs.unc.edu/techreports/89-034.pdf). You can pass in a map of the vertices to render the tree:
 
 ```js
 import { Tree, Node } from 'react-svg-tree';
 
+const vertexMap = new Map([
+  [0, [1, 2, 3]],
+  [1, [4, 5]],
+  [2, []],
+  [3, [6]],
+  [4, []],
+  [5, []],
+  [6, []],
+]);
+
 const App = () => (
   <Tree
     width={200}
     height={150}
-    rootId="O"
+    rootId={0}
     vertices={vertexMap}
     showLabels={true}
   />