This documents the input and output format, and also removes some
references to Terraform and evaluating NixOS system configurations.
It can be used to evaluate anything.
Change-Id: I8492cc3e386f89b299469c78e586644ee82a708f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11213
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
