clarify npm install step to help new npm users

Hi!

You folks have been super busy doing some really cool work lately. Over on [isaacs/npm](http://github.com/isaacs/npm) we've been getting issues opened recently from new npm users who are trying out Ghost and having a problem installing dependencies. 

It's not always clear to new users that if you run `npm install` without any arguments, it expects a package.json file, so they're opening issues on npm with the error they're getting. This is a tiny bit of clarification for new users on what to do if they receive an ENOENT error while installing Ghost's dependencies. Hopefully it can help push people in the right direction if they get stuck! (I kept it minimal because I didn't want to inflate your easy to follow instructions with a bunch of info about npm the user probably does not care about!)

Keep up the good work :)