[Chore] update README.md: added description for setting app element

update README.md: added description for setting app element.

Author: daiyanze

README.md

@@ -50,6 +50,9 @@ 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();

docs/accessibility/README.md

@@ -29,6 +29,9 @@ If you are already applying the `aria-hidden` attribute to your app content
 through other means, you can pass the `ariaHideApp={false}` prop to your modal
 to avoid getting a warning that your app element is not specified.
 
+Using `Modal.setAppElement` will not embed react-modal into your react app as
+a descendent component. It will just help boost up the app accessiblity.
+
 ### Keyboard navigation {#keyboard}
 
 When the modal is opened, it restricts keyboard navigation using the tab key to