Add docs that explain how to migrate an express app to huncwot

[emilos]

Context

It'd be helpful to describe possible migrations paths.

One idea:

const express = require('express')
const Huncwot = require('huncwot')

const app1 = express()
const app2 = new Huncwot()

app1.get('/', () => /* proxy to app2 here */)
app2.get('/', () => 'foo')

app1.listen(3333)
app2.listen(3334)

Not ideal for performance for sure, but it should make it easier to migrate apps step by step