nix: update readme (#1064)

Include the instructions for running nim-codex as a systemd service on
NixOS.

Signed-off-by: markoburcul <[email protected]>

nix/README.md

@@ -32,4 +32,28 @@ nix run 'git+https://github.com/codex-storage/nim-codex?submodules=1#''
 
 ```sh
 nix flake check ".?submodules=1#"
-```
+```
+
+## Running Nim-Codex as a service on NixOS
+
+Include nim-codex flake in your flake inputs:
+```nix
+inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+    nim-codex-flake.url = "git+https://github.com/codex-storage/nim-codex?submodules=1#";
+};
+```
+
+To configure the service, you can use the following example:
+```nix
+services.nim-codex = {
+   enable = true;
+   settings = {
+       data-dir = "/var/lib/codex-test";
+   };
+};
+```
+The settings attribute set corresponds directly to the layout of the TOML configuration file 
+used by nim-codex. Each option follows the same naming convention as the CLI flags, but 
+with the -- prefix removed. For more details on the TOML file structure and options, 
+refer to the official documentation: [nim-codex configuration file](https://docs.codex.storage/learn/run#configuration-file).