Added extra explanation for require() statements

[heysamtexas]

Just thought it would be nice to know how to use this using require() statements.

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[zpao]

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.

[heysamtexas]

👍 sounds good. Let's go with that

[zpao]

Excellent. Want to update this PR or would you prefer I just add it in? (no need for me to steal credit, it was your idea!)

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