add features in README.md

Author: rwieruch

README.md

@@ -2,7 +2,13 @@
 
 [![Build Status](https://travis-ci.org/the-road-to-learn-react/react-intersection-observer-api-example.svg?branch=master)](https://travis-ci.org/the-road-to-learn-react/react-intersection-observer-api-example) [![Slack](https://slack-the-road-to-learn-react.wieruch.com/badge.svg)](https://slack-the-road-to-learn-react.wieruch.com/)
 
-A simple application that showcases how [Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) can be used in React with [createRef()](https://reactjs.org/docs/refs-and-the-dom.html). Use case: observe sections in an article and track the current viewed section in a navigation.
+A simple application that showcases how [Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) can be used in React with [createRef()](https://reactjs.org/docs/refs-and-the-dom.html). Use case: observe sections in an article and track the current viewed section in a navigation. [Read more about it](https://www.robinwieruch.de/react-intersection-observer-api).
+
+## Features
+
+* intersection observer API
+* React's createRef to reference DOM nodes
+* intersection observer API polyfill for browsers/testing
 
 ## Installation