docs: Add documentation for nix

.config/nix/README.md

@@ -0,0 +1,28 @@
+# `null2264`'s Nix Setup
+
+You need to install Nix before following this guide: `sh <(curl -L https://nixos.org/nix/install)` and symlink this dir to `~/.config/nix`
+
+## Setup
+
+### Initial
+
+This is done because nix-darwin commands is not yet added to PATH, should be a one-time thing
+
+```zsh
+nix-env -iA nixpkgs.git
+# Run `sudo chown $USER /nix/var/nix/profiles/per-user/$USER` if that returns error
+
+nix build .#darwinConfigurations.<host>.system
+
+./result/sw/bin/darwin-rebuild switch --flake .#<host>
+# or
+nix run nix-darwin -- switch --flake .#<host>
+```
+
+### Rebuild
+
+After initial setup, you should now be able to use the command directly:
+
+```
+darwin-rebuild switch --flake .#<host>
+```