Update documention to include phpleague/flysystem

[tacman]

Is your feature request related to a problem? Please describe.

The Symfony documentation describes a configuration with oneup, but I would like use to phpleague/flysystem

*Describe the solution you'd like

https://symfony.com/doc/current/LiipImagineBundle/cache-resolver/flysystem.html

liip_imagine:
    resolvers:
        profile_photos:
            flysystem:
                filesystem_service: oneup_flysystem.profile_photos_filesystem
                root_url:           "http://images.example.com"
                cache_prefix:       media/cache
                visibility:         public

What is an example setting for

            filesystem_service: # ?? phpleague flysystem class 

[dbu]

see the first line in the configuration section of the doc: the liip-imagine flysystem resolver needs any service that is League\\Flysystem\\Filesystem. if you want to use phpleague/flysystem without a bundle, you need to define your own service with a filesystem implementation. if you use the phpleague bundle you need to figure out the service name schema that they use (i had a quick look at the doc where it seems they only describe autowiring but not how the names would look - if you want to use that bundle, you can use bin/console debug:container to see what the services are called)