Update README.md (#473)

* Update README.md

Added useState import to the example code.

* Update README.md

* Update README.md

README.md

@@ -58,6 +58,7 @@ npm install react-easy-crop --save
 > Thus, you need to wrap it with an element that uses `position: relative` or the Cropper will fill the whole page.
 
 ```js
+import { useState, useCallback } from 'react'
 import Cropper from 'react-easy-crop'
 
 const Demo = () => {