Clarified npm link docs

That wasn't clear for me that node-redis contains `package.json`
where package name is "redis", not "node-redis".

So I made easier example and added note that `package-name` lives
in `package.json` file. Hope that will save somebody 10 minutes or so.

![troy-mc-clure-1024x768](https://f.cloud.github.com/assets/89186/79080/d329bbac-61a0-11e2-9b50-7e60c82b556b.gif)