[ecash-wallet] Update README with roadmap

Summary: Need to get back into this lib, as it needs more features before it can be dropped into Cashtab (will be a big win), and more features still to simplify important onchain actions for general users.

Test Plan: proofread

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18543