chore(docs): Add Nix installation instructions

[iedame]

Added Nix package installation instructions to README.

[loathingKernel]

Thank you, merging.

Side note: Rare from the next version will be needing its own legendary fork, because upstream legendary it getting a bit outdated. My Arch PKGBUILD handles that by compiling it with Nuitka. There might be other less bloated solutions to it, but that's what I used for now. How can Nix handle it? I don't mean necessarily compiling it with Nuitka, but offering it with a Rare-specific legendary python module.

[iedame]

Thank you for the heads-up! I just started maintaining these packages after the previous maintainer left, so I'm not yet certain of the best path forward.

But it should be a simple change doing a similar approach to the PKGBUILD or starting a new package derivation with the new legendary fork to replace the old one in the rare package.

[loathingKernel]

Since the latest release, as I mentioned before, we require two packages that are not available in PYPI or generally in distributions.

This is how I handled it in my own PKGBUILDs https://github.com/loathingKernel/PKGBUILDs/tree/master/local/rare . Basically I elected to create a packaged application through Arch makepkg machinery. I am not sure a distribution would be happy with that, hopefully Nix has a better way to handle this. The two external python modules that we need are our own fork of legendary-gl and an updated fork of python-vdf, both are listed in pyproject.toml.