Why does react-modal require the prop contentLabel?
<p>I started getting this warning on my react-modal components:</p>

<blockquote>
  <p>Warning: Failed propType: Required prop <code>contentLabel</code> was not
  specified in <code>Modal</code>.</p>
</blockquote>

<p>It doesn't prevent the modal from working correctly, I just see the warning in the dev tools console. I can pass this prop in by specifying some random string, but I do not understand what this is actually used for, and why it is required.</p>