add readme

README.md

+# Node.js Language Pack
+
+The Node.js Language Pack will run `npm install` on your app after setting
+up an appropriate environment of [node](http://github.com/joyent/node),
+[npm](https://github.com/isaacs/npm) and [SCons](http://www.scons.org).
+
+## Vendored Libraries
+
+The versions of node, npm and SCons used by the language pack are controlled
+by the constants `NODE_VERSION`, `NPM_VERSION`, and `SCONS_VERSION` in
+`bin/compile`
+
+To use different versions of these tools, you can use the helper scripts
+`support/package_node` and `support/package_npm`. These support scripts will
+create binary tarballs that can be referenced by `bin/compile`.