docs(readme): add react flow instance section

example/src/Stress/index.js

@@ -3,10 +3,7 @@ import React, { useState } from 'react';
 import ReactFlow, { removeElements, addEdge, MiniMap, isNode, Controls, Background } from 'react-flow-renderer';
 import { getElements } from './utils';
 
-const onLoad = graph => {
-  console.log('graph loaded:', graph);
-  graph.fitView();
-};
+const onLoad = reactFlowInstance => reactFlowInstance.fitView();
 
 const initialElements = getElements(10, 10);