[e.cash] Update README to use npm ci

Summary:
This will install the pinned version instead of any version compliant with the package.json
It's better for reproducibility

Test Plan: Used this command, it works.

Reviewers: Fabien, johnkuney, #bitcoin_abc

Reviewed By: Fabien, johnkuney, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13924