Update README to include link to bismuth nix package

[davidpa9708]

Summary

just found bismuth on nix store, didn't add it myself

update README.md to include nix entry

https://search.nixos.org/packages?channel=23.11&show=libsForQt5.bismuth&from=0&size=50&sort=relevance&type=packages&query=bismuth

Breaking Changes

just documentation changes

UI Changes

Before	After

Test Plan

just documentation changes

Related Issues

no issues afaik