Update tutorial to include running Node server

The tutorial suggests using python -m SimpleHTTPServer to serve content, but subsequently introduces a mechanism for submitting comments via a POST request, which SimpleHTTPServer doesn't support. It appears that at this point the tutorial assumes you're using the sample Node.js server packaged with the tutorial's GitHub repo but that isn't explained.

This diff adds instructions on how to use the sample Node.js server.

Fixes https://github.com/reactjs/react-tutorial/issues/1