Reflow eCash section of the README

Summary: Reflow eCash section text so that it reads better in terminal.

Test Plan: Read and verify text contents have not changed.

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11386