I added the CDN to my code:
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css">
But when I try to add a button with a font-awesome icon, it shows a blank space:
<div class="container">
<a class="btn" href="#">
<i class="fa fa-twitter"></i>
</a>
</div>