README: add a section for test coverage
Summary:
It is a hassle to figure out the exact commands to create a good
report.
This is a backport of libsecp256k1 PR704
Depends on D5383
Test Plan: Read
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5387