Document passing arguments to dream2nix

[mightyiam]

As follow-up to #157 (reply in thread), I think that the means for passing arguments to dream2nix better be documented. I couldn't find such documentation by searching https://flake.parts/options/nix-cargo-integration for dream2nix.

[yusdacra]

i thought i already mentioned this in the drvConfig and depsDrvConfig docs but i apparently didn't, so i added a line clarifying it just now, should be up whenever flake.parts updates

[mightyiam]

🙏