docs: update README to Fastify v4 (#216)

* Update README to Fastify v4

Fixes #215

* Update README.md

* Update README.md

Co-authored-by: Uzlopak <aras.abbasi@googlemail.com>

Co-authored-by: Uzlopak <aras.abbasi@googlemail.com>

README.md

@@ -21,17 +21,19 @@ npm i @fastify/cors
 ## Usage
 Require `@fastify/cors` and register it as any other plugin, it will add a `preHandler` hook and a [wildcard options route](https://github.com/fastify/fastify/issues/326#issuecomment-411360862).
 ```js
-const fastify = require('fastify')()
+import Fastify from 'fastify'
+import cors from '@fastify/cors'
 
-fastify.register(require('@fastify/cors'), { 
+const fastify = Fastify()
+await fastify.register(cors, { 
   // put your options here
 })
 
 fastify.get('/', (req, reply) => {
   reply.send({ hello: 'world' })
 })
 
-fastify.listen(3000)
+await fastify.listen({ port: 3000 })
 ```
 You can use it as is without passing any option or you can configure it as explained below.
 ### Options
@@ -86,8 +88,10 @@ fastify.register(require('@fastify/cors'), (instance) => {
   }
 })
 
-fastify.get('/', (req, reply) => {
-  reply.send({ hello: 'world' })
+fastify.register(async function (fastify) {
+  fastify.get('/', (req, reply) => {
+    reply.send({ hello: 'world' })
+  })
 })
 
 fastify.listen(3000)