Provide unit and e2e test instructions in the top-level README

[brandonpayton]

It would be good for us to document the commands for running unit tests and E2E tests locally.

I was just going to add some instructions to the top-level README.md as a start, but it turns out there isn't a simple, single command for running E2E tests locally. The local E2E tests target a custom local domain playground.test and uses HTTPS which takes a bit more setup.