Add file detailing conventions for nix.
Add file detailing conventions for nix.

nix-rust/nix,carllerche/nix-rust,geofft/nix-rust,berkowski/nix,xd009642/nix,nix-rust/nix,xd009642/nix,posborne/rust-nix,posborne/rust-nix,MarkusJais/nix-rust,asomers/nix,posborne/rust-nix,MarkusJais/nix-rust,xd009642/nix,carllerche/nix-rust,nikklassen/nix-rust,nikklassen/nix-rust,asomers/nix,geofft/nix-rust,carllerche/nix-rust,geofft/nix-rust,kamalmarhubi/nix-rust,kamalmarhubi/nix-rust,kamalmarhubi/nix-rust,berkowski/nix,nikklassen/nix-rust,Susurrus/nix,MarkusJais/nix-rust,Susurrus/nix,berkowski/nix,Susurrus/nix