use bootstrap in gatsby code example

Example 1: add bootstrap to gatsby

npm install --save react-bootstrap
npm install --save bootstrap

Example 2: add bootstrap to gatsby

import React from 'react';
import '../path_to/bootstrap.min.css';

const MyComponent = () => (
    <div>
        <p>This contains a button which uses the bootstrap.css</p>
        <button class="btn btn-primary">Click me</button>
    </div>
);