Document how to add merkle proofing to existing interpreters

[hswick]

@OR13 asked about this in our gitter. We have a few examples of merkle proofing computation steps in ocaml-offchain, dispute-resolution-layer simple adder vm, and scrypt interactive.

Considering this is rather generic idea that applies to any instance of a truebit like system it would be great to have a nice write up on this.