Update testing directions in the readme + minor readability formatting

As discussed in the 1:1 we had yesterday (21.5), sending along minor updates to split the larger testing into one where contributors are asked to make an environment and one where the tests are explained and listed.

Included is some minor formatting that came through from format on save (Prettier). I reverted some of the changes, but I think changes like adding lines between headers and their first lines makes sense 😄

Let me know if anything else is needed!