Add high level description

[MaxHillebrand]

This is a high level summary and description of the partially blind swap protocol. It was discussed in person with @jonasnick and @AdamISZ. It does not cover the nuances of the musig creation and blinding, thus removing complexity and hopefully providing a more approachable explanation of the protocol.

[MaxHillebrand]

Thanks for the typo fixes @yahiheb and the clarification @jonasnick. Sorry for the late commits, I didn't have notifications for this repo...