Added package auto-discovery functionality

[orediggerco]

Laravel 5.5 introduced a new package auto-discovery feature to make installing Laravel packages easier. By adding extra information in the composer.json file, Laravel 5.5+ will identify the listed ServiceProvider and Facade and automatically register them without physically adding them to config/app.php in a consuming project.

I also updated the README to describe the new functionality, and modified the ServiceProviderLaravel5 class to add the group string to the publishes method.