docs: update README.md

Author: ns-vasilev

README.md

@@ -304,6 +304,7 @@ struct RegistrationView: View {
 | `CreditCardValidationRule` | Validates credit card numbers (Luhn algorithm) | `CreditCardValidationRule(error: "Invalid card number")` |
 | `EmailValidationRule` | Validates email format | `EmailValidationRule(error: "Please enter a valid email")` |
 | `CharactersValidationRule` | Validates that a string contains only characters from the allowed CharacterSet | `CharactersValidationRule(characterSet: .letters, error: "Invalid characters")` |
+| `NilValidationRule` | Validates that value is nil | `NilValidationRule(error: "Value must be nil")`
 
 ## Custom Validators