Update readme

README.md

@@ -13,14 +13,30 @@ npm install --save @zhigang1992/stateful
 ## Usage
 
 ```jsx
-import React, { Component } from 'react'
+import React, { Component, Fragment } from 'react'
 
-import MyComponent from '@zhigang1992/stateful'
+import Stateful, {Onmount} from '@zhigang1992/stateful'
 
-class Example extends Component {
-  render () {
+export default class App extends Component {
+  render() {
     return (
-      <MyComponent />
+      <Stateful state={{count: 0, seconds: 0}}>
+        {({ state, setState, getState }) => (
+          <Fragment>
+            <p>Clicked {state.count} times</p>
+            <button onClick={() => setState({ count: state.count + 1 })}>
+              Click me
+            </button>
+            <p>{state.seconds} seconds passed</p>
+            <Onmount>{() => {
+              // You can do stuff like firing a network request in here
+              window.setInterval(() => {
+                setState({seconds: getState().seconds + 1})
+              }, 1000)
+            }}</Onmount>
+          </Fragment>
+        )}
+      </Stateful>
     )
   }
 }