nixpkgs docs: improve node packages documentation

* be more specific about paths to files

pkgs/development/node-packages/README

@@ -1,13 +0,0 @@
-How to update the NPM packages
-==============================
-- Install node2nix:
-
-nix-env -f '<nixpkgs>' -iA node2nix
-
-- Modify node-packages.json, add, update or remove package entries
-
-- Run the script:
-
-sh generate.sh
-
-- Done!

pkgs/development/node-packages/README.md

@@ -0,0 +1,11 @@
+NPM packages
+===========
+ 1. Install node2nix: `nix-env -f '<nixpkgs>' -iA node2nix`.
+ 2. Modify `pkgs/development/node-packages/node-packages.json`, to add, update,
+    or remove package entries.
+ 3. Run the script: `cd pkgs/development/node-packages && sh generate.sh`.
+ 4. Build your new package to test your changes: `cd /path/to/nixpkgs &&
+   nix-build -A nodePackages.<new-or-updated-package>`. To build against a
+   specific node.js version (e.g. 5.x): `nix-build -A
+   nodePackages_5_x.<new-or-updated-package>`
+ 5. Add, commit, and share your changes!