docs: document nixos installation

Author: kamadorueda

README.md

@@ -155,6 +155,56 @@ Then run Alejandra with:
 $ alejandra --help
 ```
 
+### NixOS installation
+
+- Nix with [Flakes](https://nixos.wiki/wiki/Flakes):
+
+  ```nix
+  {
+    inputs = {
+      nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+
+      alejandra.url = "github:kamadorueda/alejandra/0.7.0";
+      alejandra.inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    outputs = {alejandra, nixpkgs, ...}: {
+      nixosConfigurations = {
+        example = nixpkgs.lib.nixosSystem rec {
+          # We support: aarch64-linux, aarch64-linux, x86_64-darwin, x86_64-linux
+          system = "x86_64-linux";
+
+          modules = [
+            {
+              environment.systemPackages = [alejandra.defaultPackage.${system}];
+            }
+            # Import your other modules here
+            # ./path/to/my/module.nix
+            # ...
+          ];
+        };
+      };
+    };
+  }
+  ```
+
+- Nix stable:
+
+  ```nix
+  let
+    alejandra =
+      (import (builtins.fetchTarball {
+        url = "https://github.com/kamadorueda/alejandra/tarball/0.7.0";
+        sha256 = "0kqkwi4j1d0n334jl3q6x70ga8jqmqpv514zsbfca2a86c7qs6cf";
+      }))
+      # Pick one from: aarch64-linux, aarch64-linux, x86_64-darwin, x86_64-linux
+      .x86_64-linux
+      .outPath;
+  in {
+    environment.systemPackages = [alejandra];
+  }
+  ```
+
 ## Do I need to configure anything?
 
 - No.