Add note regarding form type names in Symfony 3

[foaly-nr1]

When using your bundle with the minimal configuration from your docs in Symfony 3, it fails with:

InvalidArgumentException: Could not load type "text"

Configuration:

dmishh_settings:
    settings:
        my_first_setting: ~

This is because form type names were deprecated in Symfony 2.8 and removed in Symfony 3.0.

[rvanlaak]

Yes we should definitely add a note about FQCNs, but I think you can combine your change on advanced-configuration with the "Full list of options:" part at the top of that page. Maybe just add a note below that block?

     //...
     type: number # any Symfony form type, or FQCN for Symfony ^3.0

Note: Use the Fully Qualify Class Name if you use Symfony 3

     //...
     type: Symfony\Component\Form\Extension\Core\Type\NumberType # any Symfony form type

Maybe also link the note you've put at installation.md to the advanced-configuration, so we have one single place with details about this?

[foaly-nr1]

@rvanlaak ready for review.

[rvanlaak]

Thanks @foaly-nr1