Bring README logo graphic into repo

Summary: May as well not use bandwidth from our website to serve the image on Github

Test Plan: Tested that it works at: https://github.com/Mengerian/bitcoin-abc/tree/readme-logo

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9717