[chore] readd setAppElement example

Author: neilhsmith

README.md

@@ -123,6 +123,7 @@ There are several demos hosted on [CodePen](https://codepen.io) which
 demonstrate various features of react-modal:
 
 * [Minimal example](https://codepen.io/neilhsmith/pen/QWVVEEg)
+* [Using setAppElement](https://codepen.io/neilhsmith/pen/JjmoaOV)
 * [Using onRequestClose](https://codepen.io/neilhsmith/pen/OJooXBg)
 * [Using shouldCloseOnOverlayClick](https://codepen.io/neilhsmith/pen/wvEYxKa)
 * [Using inline styles](https://codepen.io/neilhsmith/pen/gOdZPPP)

docs/examples/set_app_element.md

@@ -0,0 +1,7 @@
+# Using setAppElement
+
+This example shows how to use setAppElement to properly hide your application from screenreaders and other assistive technologies while the modal is open.
+
+You'll notice in this example that the aria-hidden attribute is applied to the #root div rather than the document body.
+
+[setAppElement example](https://codepen.io/neilhsmith/pen/JjmoaOV)