Document `preserveStringified` on `getValidatedFormData`

[KarstenBuckstegge]

The third argument of getValidatedFormData is not described in the documentation. Therefor a string value that only contains numbers being passed into getValidatedFormData is automagically converted to a number, causing unexpected validation errors.

Documenting the preserveStringified attribute will explain that behaviour.

[timvandam]

I encountered this as well. Also wonder why the defaults expect everything to be json. That does not make much sense to me as pretty much all forms I implement are just raw text or numbers that zod will coerce for me

[AlemTuzlak]

I documented the behavior in the readme!