Add new grouping example

[exoego]

When I migrated my app from Express to Hono, I needed this example.

Express

const book = express()
book.get('/book', ...)
book.post('/book', ...)

const user = express()
user.get('/user', ...)
user.post('/user', ...)

const app = user.use(book) // no base path needed

Hono

const book = new Hono()
book.get('/book', ...)
book.post('/book', ...)

const user = new Hono()
user.get('/user', ...)
user.post('/user', ...)

const app = user.route('/', book)

[yusukebe]

Nice! Thanks @exoego