Add links to node and npm in Requirements

In getting started, it asks if you are new to node and npm, but there are no links to explain what either is. I believe these links help to clarify.

docs/GettingStarted.md

@@ -12,7 +12,7 @@ next: tutorial
 1. OS X - This repo only contains the iOS implementation right now, and Xcode only runs on Mac.
 2. New to Xcode?  [Download it](https://developer.apple.com/xcode/downloads/) from the Mac App Store.
 3. [Homebrew](http://brew.sh/) is the recommended way to install node, watchman, and flow.
-4. `brew install node`. New to node or npm?
+4. `brew install node`. New to [node](https://nodejs.org/) or [npm](https://docs.npmjs.com/)?
 5. `brew install watchman`. We recommend installing [watchman](https://facebook.github.io/watchman/docs/install.html), otherwise you might hit a node file watching bug.
 6. `brew install flow`. If you want to use [flow](http://www.flowtype.org).