Update readme

- Mention supported/tested selenium/browser and any version-specific problems
- Installation (standard composer stuff)
- Usage
    - [x] in behat
    - [x] in mink
    - [x] individually
- How to run tests
    - individually
        - with docker
        - with native selenium (+mention custom phpunit config)
    - in bulk with `act`
- Tests to ensure readme examples are not broken