Docs should mention Cabal 2.4 is needed

[puffnfresh]

I was trying to figure out why plan-to-nix wasn't generating "extras" for me. I had a look at the code and realised "pkg-src" didn't exist in my plan.json and that's because my version of Cabal was old.

Docs mentioning Cabal 2.4 would be useful.

[puffnfresh]

Should this issue go under nix-tools?

[rvl]

Thanks for the report. Yes this is the correct place to submit nix-tools issues.

I'm updating the docs just now in #165 by the way.