Missing example for device_code use case

When trying to get started with this library it all seems very much biased towards web apps or API token use cases.

I would like to see documentation using [device_code](https://developers.google.com/identity/protocols/oauth2/limited-input-device) authentication.

Basically using the `https://oauth2.googleapis.com/device/code?client_id=${CLIENT_ID}&scope=${SCOPE}` endpoint.

It isn't hard to implement by hand, but then the entire library is worthless anyway because its easier to just use the rest endpoints to begin with.