[Cashtab] Update README to advise of local dependencies

Summary: Not necessarily clear to new devs that these must be built before Cashtab will run locally. Previously npm i was enough, since they are built as hosted by npmjs.

Test Plan: Proofread

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16271