README should include info about Rhino

[lancecarlson]

I spent a little bit of time trying to figure out why things weren't working and it all stemmed from the fact I had an older version of Rhino installed on my OS X computer inside of ~/Library/Java/Extensions/. There error was:

org.mozilla.javascript.WrappedException: Wrapped java.lang.IllegalArgumentException: Bad language version: 180

Perhaps specifying that the preleased version of Rhino is required and that it is included by default when you run narwhal would be helpful. It would've gotten me up and running faster. :)