Update CONTRIBUTING.md to include Simplecov

[corsonknowles]

Add Simplecov instructions, common usage, and the repo link to our CONTRIBUTING.md guidelines.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.

[corsonknowles]

@bquorning How does this look?

Amendments and improvements welcome.

[bquorning]

Do you have mdformat available locally? mdformat . should fix the CI.

[corsonknowles]

I can do that! It will be a few hours until I am back online today.

[corsonknowles]

Formatted!

[bquorning]

Yeah, but… Maybe we should merge #2037 first?

[corsonknowles]

Good call! This latest amendment should get it passing enough to merge in either order.