Add Example usage with react-native-vector-icons

[okwasniewski]

React Native Vector Icons allow users to retrieve the source of the icon.

Goal of this issue is to add new example & section in docs to outline how to use React native vector icons:

import Icon from 'react-native-vector-icons/Ionicons';

const navIcon = Icon.getImageSourceSync('md-arrow-back', 24, 'white');
const overflowIcon = Icon.getImageSourceSync('md-more', 24, 'white');
const settingsIcon = Icon.getImageSourceSync('md-settings', 30, 'white');
const twitterIcon = Icon.getImageSourceSync('logo-twitter', 25, '#4099FF');

[fredrikburmester]

How can I use @expo/vector-icons?