julia: add README
julia: add README

Provides a few hopefully helpful pointers that would not work well as
inline comments in the expressions themselves. Most likely the README
will need to be expanded upon over time to cover how we handle the Julia
release process, but I hope this is a good starting point.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs