describe compatibility in readme - fixes #24

[malcomio]

one problem here is that 1.2.1 uses "phpro/grumphp": "^0.16",, which means that we see the problem mentioned in https://github.com/vijaycs85/drupal-quality-checker/pull/22#issuecomment-590217149

[vijaycs85]

Thanks, @malcomio we are now at 1.14 (#34) and the version constraints have been removed now.