Update module on Drupal.org

[DieterHolvoet]

Description

We should merge this version with the one on Drupal.org and provide a clear upgrade path.

• Add upgrade docs for when you're coming from the Drupal.org version of the module
• Become maintainer of the Drupal.org project (instructions)

After becoming maintainer:

• Change the project description, take the old README with a contribution section (example) and update the composer install command
• Remove inactive maintainers from the Drupal.org project
• Clean up the Drupal.org issue queue
• Archive this repository, make it point to the new one
• Abandon the package on Packagist and put drupal/imgix as replacement (example)