title: Best way to polyfill ES6 features in React app that uses create-react-app
body: <p>I've been testing my React.js application on internet explorer, and finding that some ES6/7 code like <code>Array.prototype.includes()</code> breaks it. </p>
  
  <p>I'm using <a href="https://github.com/facebookincubator/create-react-app" rel="noreferrer">create-react-app</a>, and apparently they've chosen not to include a lot of polyfills since not everyone needs them, and they slow down build times (see for example <a href="https://github.com/facebookincubator/create-react-app/issues/942" rel="noreferrer">here</a> and <a href="https://github.com/facebook/create-react-app/issues/914#issuecomment-255336818" rel="noreferrer">here</a>). The documentation (at time of writing) suggests:</p>
  
  <blockquote>
    <p>If you use any other ES6+ features that need runtime support (such as
    Array.from() or Symbol), make sure you are including the appropriate
    polyfills manually, or that the browsers you are targeting already
    support them.</p>
  </blockquote>
  
  <p>So... <strong>what is the best way to 'manually' include them?</strong> </p>