docs: add section on testing style that we promote (#1645)

[clemens]

As outlined in my comment in https://github.com/thoughtbot/shoulda-matchers/issues/1645, this adds a section that explains the implementation-based testing style that shoulda-matchers promotes over behavior-/outcome-based style for the sake of providing clarity to newcomers to the gem, especially less experienced engineers.

I've added it to the "Usage" section. It seemed appropriate there because this section also addresses the topic of should vs. is_expected.to which, to me, is a similar stylistic topic where opinions and promoted practices might differ.

I'm absolutely open to feedback and adapting individual phrases as well as adding/removing parts as needed -- just let me know.