Add test coverage to Readme

[jackreichert]

Context

As this project is built we want to ensure that our quality doesn't lack. A badge on the readme page is one way to help do this.

As an engineer I want to see what the test coverage is at a glance So that I can keep my code quality at the highest standard

Acceptance Criteria

• test coverage should run in the pipeline
• output should be converted to a badge
• readme should be updated with latest badge

[jackreichert]

Resolved by https://github.com/jackreichert/over-engineered-todo/pull/33