Modify README.md to clarify setup steps.

Incorporating feedback from https://bugzilla.mozilla.org/show_bug.cgi?id=936760. 

> Step 3 installs step 4 (install npm) I think.  Recent versions of node ship with it.  Maybe we should just specify node 0.10.x.  (We probably still work on 0.8.x, but I'm not sure why we'd want to advocate an older version.)

At least on OS X and My ubuntu VM, I had to install npm separately using brew / apt-get after installing node.js. Maybe if a person downloads the source and compiles it installs automatically? My OS X Node is 0.10.21 and my Ubuntu version is v0.10.15.