[cashtab-components] Updating readme

Summary:
T2760

Update the README to reflect the Bitcoin ABC repo and accurate URLs. Replace BCH addresses with XEC addresses.

Test Plan: Confirm changes are only to the README and package.json. Sanity check read of changes.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12384