Add DoctrineStorage to the documentation

[Ruesa18]

References: #377 and #377 (comment) in particular

This PR adds a README entry on how to configure the DoctrineStorage in a project.

[RafaelKr]

I would recommend to split the configuration if you are using Symfony 4.0+ to better comply with Symfony standards (https://symfony.com/doc/current/bundles/configuration.html#using-the-bundle-extension):

# config/packages/craue_form_flow.yaml
services:
    Craue\FormFlowBundle\Storage\UserSessionStorageKeyGenerator:
        arguments: [ '@security.token_storage', '@request_stack' ]
    Craue\FormFlowBundle\Storage\DoctrineStorage:
        arguments: [ '@doctrine.dbal.default_connection', '@Craue\FormFlowBundle\Storage\UserSessionStorageKeyGenerator' ]
    myCompany.form.flow.storage.doctrine_storage:
        class: 'Craue\FormFlowBundle\Storage\DataManager'
        arguments: [ '@Craue\FormFlowBundle\Storage\DoctrineStorage' ]

# config/services.yaml
services:
    myCompany.form.flow.createVehicle:
        autoconfigure: false
        calls:
            - [ setDataManager, [ '@myCompany.form.flow.storage.doctrine_storage'] ]
            - [ setFormFactory, [ '@form.factory' ] ]
            - [ setRequestStack, [ '@request_stack' ] ]
            - [ setEventDispatcher, [ '@?event_dispatcher' ] ]

[Ruesa18]

Thanks @RafaelKr
I added your code snippet 👍