Update README.md for test and MySQL migration

[e-wai]

Implementation description

• This is technically ahead of its time (references change that will be made in Add pytest to backend #63) but well 🤭

Steps to test

1. Not really, I just wanted to play with in-browser VScode

What should reviewers focus on?

• Makes sense? Missing anything?

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• For backend changes, I have run the appropriate linters: docker exec -it <backend-container-id> /bin/bash -c "black . && isort --profile black ."
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR