Readme improvements

[lucaboesch]

Show the README.md page with some pills.

[timhunt]

Why is the change LICENSE.md -> LICENSE good?

It is confusing to add a Behat test in this commit / pull request. Should that be submitted separately? (It is a good test - except I would have it run on all Moodle versions, to show that the expected behaviour in this scenario has not changed - I don't see why there are currently two scenarios. And, if you need a comment as the first thing in a scenario, you should probably improve the scenario name instead.)

[lucaboesch]

In order to make the license pill [![License GPL-3.0](https://img.shields.io/github/license/timhunt/moodle-availability_quizquestion?color=lightgrey)](https://github.com/timhunt/moodle-availability_quizquestion/blob/main/LICENSE)
working in my experience. It wouldn't work with LICENSE.md. See studentquiz.