title: Where should ReactDOM be imported from?
body: <p>After upgrading to version <code>0.14.2</code>, I see an error and recommendation to use <a href="https://facebook.github.io/react/docs/top-level-api.html#reactdom"><code>ReactDOM.render()</code></a> instead of <code>React.render()</code>, but whence do I import it?</p>
  
  <p>When I don't import it and just running as it is, it shows it as <code>undefined</code>. Is it a built-in functionality or is it a 3rd party library?</p>