Download the example or clone the repo:

curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/custom-server
cd custom-server

Install it and run:

npm install
npm run dev

Deploy it to the cloud with now (download)

now

This example demonstrates how to serve files such as /robots.txt and /sitemap.xml from the root.