Explain somewhere how to work with `Js.t` types

[jchavarri]

I don't think the docs mention the {. "foo": string} syntax for types, looking at https://melange.re/v2.1.0/communicate-with-javascript/#using-jst-objects.

Also should include some explanation for the Reason syntax feature, where {. "box": string} becomes a shortcut for Js.t({. box: string}).