Document Function Signature for Struct-Level Validation

[JoelDavisEngineering]

Please document the function signature for Struct-Level validation.

While the 'Stuct level validation' section in the README.md shows how to declare the use of a custom function, it does not detail the function signature, itself. The programmer is left to guess that the function takes an instance of the Struct being validated.

#[derive(Debug, Validate, Deserialize)]
#[validate(schema(function = "validate_category", skip_on_field_errors = false))]
struct CategoryData {
category: String,
name: String,
}

Thanks