add nix build instructions and README section

[sheepforce]

This adds Nix build instructions and a README section on its usage.

[scemama]

@sheepforce Thanks a lot!
I see that in your nix file you get trexio from git clone, so there is a dependency to emacs and swig. This dependency is not required if you compile trexio from the tar.gz instead of the git cloned repository. Is it possible for you to modify the nix package so that it fetches the tar.gz on the release page instead?

[sheepforce]

I see that in your nix file you get trexio from git clone, so there is a dependency to emacs and swig.

You mean in upstream nix here: https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/by-name/tr/trexio/package.nix#L16
or in the Nix files in this PR here https://github.com/TREX-CoE/trexio/pull/173/files#diff-ff58cc3517012ebf72873a9638bd18d5acb9b8f9d9032c2df4d98c861bf3ebe6R16 ?

For upstream Nix we had a short discussion in the PR about that and decided to use the Git sources directly. It plays nicer with the fetchers and Nix is a source oriented package manager, where we often try to by as close to the source as possible. For example, we also run the libint generator and do not only build the libint release tarballs with generated code. In principle, it would of course be possible to use the release tarballs directly, but it would not really reduce dependencies. Emacs and Swig are in nativeBuildInputs and thus not part of the runtime closure: https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/by-name/tr/trexio/package.nix#L30

For the second case, i.e. the flake.nix in this PR, Nix is using the code of the git checkout, the flake.nix itself is in (sorry if this sentence is confusing. I couldn't find a better wording 😅) I.e. you instruct Nix to build a specific git reference, Nix will build exactly that specific git reference, using the flake.nix contained in that commit. For example nix build github:trex-coe/trexio/madelung would build the code of the madelung branch (if this PR would already be merged and part of the madelung branch). Thus, in this case, it doesn't really make sense to fetch the release tarball. This is basically the overarching goal of flakes: You include a flake.nix in all of your repos and use git references to build exactly that checkout, e.g. in another repo

{
  inputs.trexio.url = "github:trex-coe/trexio/madelung";
  outputs = { trexio, ... }: # ...
}

[scemama]

OK, I get it! Thanks for the details.

[q-posev]

Thank you @sheepforce ! Looks good to me.