I had the same problem. Read their Readme.md, and you see that there is this note:
Note: This component does not include any of the Font Awesome CSS or fonts, so you’ll need to make sure to include those on your end somehow, either by adding them to your build process or linking to CDN versions.
So the most simple way is to link to the fontawesome cdn in your html.
<head>
<meta charset="UTF-8">
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-
awesome.min.css" rel="stylesheet" integrity="sha384-
wvfXpqpZZVQGK6TAh5PVlGOfQNHSoD2xbE+QkPxCAFlNEevoEH3Sl0sibVcOQVnN"
crossorigin="anonymous">
</head>