Scilla style guide

[jubnzv]

After introducing scilla-fmt, we should describe the recommended Scilla style guide used by the formatter.

Changes in Zilliqa/scilla#1172 add a special meaning to some of the comments at the top level:

scilla_version 0
(* File comment that describes what this contract module does. *)
import BoolUtils

(* Library comment *)
library ExampleLib

(* Library function comment *)
let a = Uint32 1

(* Contract comment *)
contract ExampleContr

(* Component comment *)
procedure foo()
end

This technical decision is required for the implementation, and it allows us to do something interesting using this comments structure. For example, we may want to implement an ocamldoc-style documentation generator or something like this.

So, we should somehow document where these comments should be placed and their meaning.