[eCash dev libraries] Improve README for ecashjs-lib

Summary:
T2847

Add rev one README for ecashjs-lib. Room for improvement here, but at the moment we just need something to start from.

Test Plan: Review README.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D12884