Add install instructions for Nix / NixOS

[sharkdp]

@dywedir It looks like you maintain(?) the hyperfine package for nix?

I would like to add install instructions to the README. Would this be:

nix-env -i hyperfine

[dywedir]

Hi! Not an "official" one — nixpkgs open to contribution, so I'm doing some maintenance :)

nix-env -iA nixos.hyperfine

— seem to be a better option

[sharkdp]

Thanks!

[dywedir]

seem to be a better option

This is because nix-env -i hyperfine is too slow.
But nix-env -iA nixos.hyperfine is for NixOS channel only, so you can also add variant for Nix on other platforms: nix-env -iA nixpkgs.hyperfine.