nodePackages: add more details about the packages to include and the Node.js versions to support

pkgs/development/node-packages/README.md

···
 Node.js packages
 ================
-To add a package from [NPM](https://www.npmjs.com/) to nixpkgs:
+The `pkgs/development/node-packages` folder contains a generated collection of
+[NPM packages](https://npmjs.com/) that can be installed with the Nix package
+manager.
+
+As a rule of thumb, the package set should only provide *end user* software
+packages, such as command-line utilities. Libraries should only be added to the
+package set if there is a non-NPM package that requires it.
+
+When it is desired to use NPM libraries in a development project, use the
+`node2nix` generator directly on the `package.json` configuration file of the
+project.
+
+The package set also provides support for multiple Node.js versions. The policy
+is that a new package should be added to the collection for the latest stable LTS
+release (which is currently 6.x), unless there is an explicit reason to support
+a different release.
+
+To add a package from NPM to nixpkgs:
 
  1. Modify `pkgs/development/node-packages/node-packages-v6.json` to add, update
     or remove package entries. (Or `pkgs/development/node-packages/node-packages-v4.json`
-    for packagages depending on Node.js 4.x)
+    for packages depending on Node.js 4.x)
  2. Run the script: `(cd pkgs/development/node-packages && ./generate.sh)`.
  3. 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. 4.x):
     `nix-build -A nodePackages_4_x.<new-or-updated-package>`
  4. Add and commit all modified and generated files.
+
+For more information about the generation process, consult the
+[README.md](https://github.com/svanderburg/node2nix) file of the `node2nix`
+tool.