Point to `util.promisify` in README?

[courajs]

Node 8 introduced util.promisify to the standard library, which handles the simple use case of this library. It also allows functions to opt-in to specifying how they are promisified, which means you can promisify even some non-node-style callback functions (like setTimeout). Can we add a note in the readme stating if they use Node 8, they may want to use it instead?