Document extension nodes

[jchavarri]

I am missing a section that I could link to from melange.re where it explains how extension nodes are parsed and rendered in Reason.

https://v2.ocaml.org/manual/extensionnodes.html

In particular, the differences between extension nodes used with expressions:

let add = [%bs.raw "a + b"];

vs used with structures and signatures:

%bs.raw "var a = 1; var b = 2";

I guess this new section could also document attributes, as they are closely related to extensions.