Fixed Drupal core PHPStan rule classes not found by Composer autoload…

…er. Fixes #33.

Added installation instructions for use with justafish/ddev-drupal-co…

…re-dev.

Changed wording.

Changed version of symfony/var-dumper to be lenient.

Fixed ‘drush cr’ command replacement on Drush 12. Not compatible with…

… Drush 11.

Added sample phpunit.xml file for use on DDEV.

Added Composer script to allow contrib modules to be installed on 11.x. 

Fixes #23.

Added roadmap section to README.

Added DDEV instructions.

Removed unused imports.