README: add a section for test coverage
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 [[https://github.com/bitcoin-core/secp256k1/pull/704 | 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

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