[ci skip] add tip for uglifier in execjs error

I'm not sure we need to add "Usually Mac OS X and Windows come with a JavaScript runtime installed."

Also I would rephrase the first sentence as:

> Compiling CoffeeScript and JavaScript asset compression require you have a JavaScript runtime available on your system, in the absence of a runtime you will see an `execjs` error during asset compilation.

Notice I left out the part about uglifier, as this is kind of a minor detail we can cover under asset pipeline. It's not important for getting started.