[marlin-website] Add a README

Summary:
This has been requested. There is not much to say, so we don't say much.

Depends on D19027.

Test Plan: Read it. Test it. Enjoy it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19028