[docs] Add instructions for lcov coverage report generation

Summary:

After rediscovering the lcov report generation recipe one too many
times, it seemed prudent to write some doc.

Backport of core PR11680
https://github.com/bitcoin/bitcoin/pull/11680/files

Depends on D3943.

Test Plan: Read the doc and try the instructions.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3944