Added extra explanation for require() statements

Just thought it would be nice to know how to use this using require() statements.
We do mention it super briefly in the npm readme, but it's not obvious. So let's do it. I'd like to tweak the wording though (you *have to* use require in Node, so there's no if about it).

Maybe something along the lines of this?

> When using the `react` package in from npm, you can simply `require('react/addons')` instead of `require('react')` to get `React` with all of the addons.