Update code coverage and quality guidance

[adrielp]

Overview

Update the code coverage section to be more verbose and clear. Code coverage is not a set of absolute truths. There is no number that guarantees quality. It should be made clear on some example of why code coverage can be useful, and where it's not useful.

Things to consider:

• mvps vs production grade (showing quick value vs long term maintenance of a production application)
• 80 % might be ok, 5 % might also be ok
• If all your tests are mocks your code still sucks.

[adrielp]

Marking this as won't do because it's been open so long and we have other priorities right now. We can revisit later.