Rewrite README intro to be more objective

Summary:
A more objective intro sounds more confident. The end was also reworded to reflect
existing eCash branding and describe the goal instead of a particular state in time.

Test Plan: read

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Mengerian, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Mengerian, Fabien

Subscribers: Mengerian, bytesofman, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11381