Document how to install dependencies

[gaearon]

Per @ForbesLindesay’s suggestion, we should add a section on installing dependencies into the howto. This is a common source of confusion: #182 #207.

A PR is welcome! We should recommend npm install --save <library-name>.
(But please comment here that you’re working on it.)

[danthareja]

@gaearon - I'll take this one! PR coming soon