Add instructions to view the test coverage reports locally

[seisman]

Description of the desired feature

The Contributing Guidelines says:

Run the tests and calculate test coverage using:

make test

The coverage report will let you know which lines of code are touched by the tests. Strive to get 100% coverage for the lines you changed.

However, it's not clear how to view the coverage reports locally. We should add instructions to do that and perhaps add a make target, e.g., make view-coverage.

Are you willing to help implement and maintain this feature? Yes/No