[Peers] Add doc/transport_module.md

[julianknutsen]

As a new dev to the transport layer, I want to quickly read about the objects, interfaces, and tests so I can understand the code and add features with minimal ramp up time.

Definition of Done:

• Add additional documentation about the separate transport layer, the interfaces, objects, and potential use cases in the future.

Tech Discussion:
This separate module could have uses outside of peers. Document it once the new object model is in and include the potential use cases. Suggested here.