readme is not correct

``` php
    $cardRegistration = new CardRegistration();
    $this->get('troopers_mangopay.payment_helper')->createPreAuthorisation($cardRegistration);
```

as createPreAuthorization does not take a cardRegistration, i guess you meant:

``` php
 $this->get('troopers_mangopay.card_registration_helper')->createCardRegistrationForUser($user);
```
