docs: update README.md

Author: ns-vasilev

README.md

@@ -303,6 +303,7 @@ struct RegistrationView: View {
 | `URLValidationRule` | Validates URL format | `URLValidationRule(error: "Please enter a valid URL")` |
 | `CreditCardValidationRule` | Validates credit card numbers (Luhn algorithm) | `CreditCardValidationRule(error: "Invalid card number")` |
 | `EmailValidationRule` | Validates email format | `EmailValidationRule(error: "Please enter a valid email")` |
+| `NilValidationRule` | Validates that value is nil | `NilValidationRule(error: "Value must be nil")`
 
 ## Custom Validators