Add note to Github repo for easier cloning of development source tree

Summary: Adds a link to the git repo clone URL of the active development repo.

Test Plan: review README change

Reviewers: #bitcoin_abc, deadalnix, CCulianu, HostFat

Reviewed By: #bitcoin_abc, deadalnix, CCulianu, HostFat

Differential Revision: https://reviews.bitcoinabc.org/D509