[ZIP 324] Specify Liberated Payments

[daira]

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[jackgavigan]

Leaving some links here as reminders: https://docs.google.com/document/d/19I45TlOQo5kxvFhgDXPuLUWq07Hy5mU2ER1s8UWT2gk/edit

zcash/zcash#801