I have this simple index :
If you want to include css in your html, you gotta put them in your public folder and use '%PUBLIC_URL%' as the path to your css. So it would look something like this:
Alternatively, it is highly recommended that you do import the css that you want inside your react component, because your css would get minified and bundled. So you could do something like:
import React, { Component } from 'react';
// ... rest of imports
import './path/to/css/font-awesome.min.css'
For more info on this, go to the documentation here.