[Cashtab] Update out of date readme

Summary:
T2826

A good portion of the README in Cashtab is obsolete. Update obsolete parts.

Test Plan: Review README and advise any further recommended changes.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D13549