Provide recommendations for Ruby Gem test code coverage analysis

[jrgriffiniii]

This follows #126, as it may still be valuable to retain the knowledge acquired when working with the coveralls service and https://github.com/simplecov-ruby/simplecov. Further, documenting how the code coverage reports may be uploaded to CircleCI as artifacts (please see https://github.com/samvera/samvera-circleci-orb/blob/main/src/commands/parallel_rspec.yml#L8) for access may also be valuable.

[cjcolvar]

FWIW Avalon uses code climate for coverage reporting. I extracted a version of it some time ago into a WIP PR to the orb: samvera/samvera-circleci-orb#5