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