Create READMEs per module

[HDauven]

Summary

When looking into Rusk, it's not immediately clear how all the modules connect. These modules also lack any further documentation about what the module entails and how it relates to other modules.

Possible solution design or implementation

Create READMEs per module that explain the basics of what it does, how to use it and how it connects to other modules.

The root README should also be updated with the make commands available.

It would also be helpful to explain more about the contracts by adding READMEs there, like the transfer contract being the main entrypoint for both Moonlight and Phoenix transactions.