Add currency description (prevent confusion between uSTX & Satoshis)

[zone117x]

Would be helpful to use a common format when describing amounts of currency, potentially even Rossetta's schema, as to avoid satoshis getting confused with uSTX.

Originally posted by @kyranjamie in https://github.com/blockstack/stacks-blockchain-api/issues/331#issuecomment-728221177

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[smcclellan]

Closing due to lack of activity.