docs: add instructions for installing nix

Author: obycode

contrib/nix/README.md

@@ -4,6 +4,11 @@ Build `stacks-node` and `stacks-signer` by pointing to the `flake.nix` file in
 this directory. For instance, from the root directory: `nix build
 './contrib/nix'`.
 
+## Installing `nix`
+
+Follow the [official documentation](https://nix.dev/install-nix) or use the
+[Determinate Nix Installer](https://github.com/DeterminateSystems/nix-installer).
+
 ## Using `direnv`
 
 If using `direnv`, from the root directory of this repository: