Prettify README title

Summary: This will make the README title render with a nice-looking graphic in Github.

Test Plan:
Have a look at: https://github.com/Mengerian/bitcoin-abc/tree/readme-logo
Scroll down to see the README
Hover over logo with mouse, see that "Bitcoin ABC" tooltip appears
Click on logo, it should take you to the bitcoinabc.org website
Check that alt text with broken image link works at:
https://github.com/Mengerian/bitcoin-abc/blob/readme-logo-2/README.md

Reviewers: deadalnix, #bitcoin_abc, jasonbcox, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: Fabien, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3513