Document Nix basics as it pertains to Haskell development

[srid]

We want to provide a list of "recipes" (based on https://github.com/srid/haskell-flake ) like,

• How to add a dependency (add to .cabal file, and relaunch the nix shell)
  • How to override them in Nix (eg: to work with a fork)
• How to override cabal flags for an existing dependency
• How to specify GHC version; https://github.com/srid/haskell-template/discussions/60#discussioncomment-4684053

TBD on top of #39

[srid]

https://github.com/NixOS/nixpkgs/blob/master/doc/languages-frameworks/haskell.section.md

[srid]

https://haskell.flake.page/basics