deliver documentation does not explain Deliverfile

[The docs for deliver](https://docs.fastlane.tools/actions/deliver/) mention the Deliverfile in passing, but the page does not ever introduce the concept anywhere or have any examples / instructions on how to set one up.

Went through the documentation my first time using fastlane and didn't even know there was an option to use a Deliverfile, and only realized it was a thing because of other Github issues posted on the fastlane repo.