Use ES6 syntax in README

The implicit equivalent to using our library is
`require('adorable-avatars').default`, but we're not really going to
highlight/encourage that. Babel will smooth over this bump if you're
using it.

Author: rylnd

README.md

@@ -17,10 +17,10 @@ Then, use the router middleware within your application:
 
 ```js
 // your_server.js
-var express = require('express');
-var avatarsMiddleware = require('adorable-avatars');
+import express from 'express';
+import avatarsMiddleware from 'adorable-avatars';
 
-var myApp = express();
+const myApp = express();
 myApp.use('/myAvatars', avatarsMiddleware);
 ```