update readme

README.md

@@ -1,5 +1,25 @@
 # nix-npm-overlay
 
-Overlay to fix Nix versions of NPM, PNPM, and Yarn.
+This repository is a Nix overlay for NPM, PNPM, and Yarn. It is used in [Nixpacks](https://github.com/railwayapp/nixpacks).
 
-This is used in [Nixpacks](https://github.com/railwayapp/nixpacks).
+Nix derivations for these package managers are automatically generated for the latest versions.
+
+## Usage
+
+```nix
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  npmOverlay = import (builtins.fetchTarball "https://github.com/railwayapp/nix-npm-overlay/archive/main.tar.gz");
+  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz") {
+    overlays = [ npmOverlay ];
+  };
+in
+pkgs.buildEnv {
+  name = "npm-overlay-env";
+  paths = with pkgs; [
+    nodejs_20
+    pnpm-9_x
+  ];
+}
+```