Add docs on snapshotting plugins

[mablae]

your_snapshot_plugin:
       class: Prooph\Snapshotter\SnapshotPlugin
       factory: [Acme\EventStoreSnapshotPluginFactory, createPlugin]
       arguments: ['@prooph_service_bus.your_command_bus']
       tags:
             - { name: prooph_event_store.your_event_store.plugin }

From gitter: (@sandrokeil)
@itcraftsmanpl You can register the snapshotter as a plugin https://github.com/prooph/service-bus-symfony-bundle/blob/master/doc/getting_started.md

[akondas]

@mablae thanks, save my day 👍

[mablae]

@itcraftsmanpl I just wanted to safe it here as a reminder for us to add to docs