Document the DFLang syntax

[JustusAdam]

The syntax used by the DFLang parser in the test-tools package should be documented somewhere.

The syntax currently looks like the following:

(* comments work just like they do in alang *)
let v = some.package/sf0<0> (* function id, required *) (arg1, $4 (* env arg *)) in
let (a, b) (* destructuring *) = dataflow (* for a dataflow function, rather than a stateful function *) ohua.lang/collect<1>(size, elements) in
let c = some.pacakge/sf1<2> () (* function with no arguments *) [size] (* optional context arg *) in
returnVar