Document credit-card constraints

[wouterj]

Symfony2.2 includes some new constraints on validating credit-card numbers: symfony/symfony#4734 and symfony/symfony#5072

[weaverryan]

The LuhnValidator is already documented, but the CardScheme validator is definitely needed.

[Sgoettschkes]

I can do this! Any ideas on how detailed it should be? I don't think there needs to be a list of the regexes, but maybe a list of the supporter CreditCard companies?

[Sgoettschkes]

@weaverryan This one can be closed I guess?