[Chore] Update README (example with React Hooks).

Author: nagayev

README.md

@@ -53,46 +53,35 @@ const customStyles = {
 // Make sure to bind modal to your appElement (http://reactcommunity.org/react-modal/accessibility/)
 Modal.setAppElement('#yourAppElement')
 
-class App extends React.Component {
-  constructor() {
-    super();
-
-    this.state = {
-      modalIsOpen: false
-    };
-
-    this.openModal = this.openModal.bind(this);
-    this.afterOpenModal = this.afterOpenModal.bind(this);
-    this.closeModal = this.closeModal.bind(this);
+function App(){
+  var subtitle;
+  const [modalIsOpen,setIsOpen] = React.useState(false);
+  function openModal() {
+    setIsOpen(true);
   }
 
-  openModal() {
-    this.setState({modalIsOpen: true});
-  }
-
-  afterOpenModal() {
+  function afterOpenModal() {
     // references are now sync'd and can be accessed.
-    this.subtitle.style.color = '#f00';
+    subtitle.style.color = '#f00';
   }
 
-  closeModal() {
-    this.setState({modalIsOpen: false});
+  function closeModal(){
+    setIsOpen(false);
   }
 
-  render() {
     return (
       <div>
-        <button onClick={this.openModal}>Open Modal</button>
+        <button onClick={openModal}>Open Modal</button>
         <Modal
-          isOpen={this.state.modalIsOpen}
-          onAfterOpen={this.afterOpenModal}
-          onRequestClose={this.closeModal}
+          isOpen={modalIsOpen}
+          onAfterOpen={afterOpenModal}
+          onRequestClose={closeModal}
           style={customStyles}
           contentLabel="Example Modal"
         >
 
-          <h2 ref={subtitle => this.subtitle = subtitle}>Hello</h2>
-          <button onClick={this.closeModal}>close</button>
+          <h2 ref={_subtitle => (subtitle = _subtitle)}>Hello</h2>
+          <button onClick={closeModal}>close</button>
           <div>I am a modal</div>
           <form>
             <input />
@@ -104,7 +93,6 @@ class App extends React.Component {
         </Modal>
       </div>
     );
-  }
 }
 
 ReactDOM.render(<App />, appElement);