Add a comment to the output with what inputs was the file generated

[domenkozar]

Something like:

# Generated by stack2nix 0.1 using: 
# --revision: 3d0e1e41873b4eacff1fcb153d04c2fa8a7dc467 https://github.com/input-output-hk/cardano-sl.git
...

[domenkozar]

I also wonder if we should include sha256sum of stack.yaml.