Document handler attribute validation, transactional handlers and registry reset

[papac]

Summary

Readme-only change. Documents three behaviours of the CQRS package that were implemented but not described anywhere:

• Handler attribute validation. A class passed to CQRSRegistration::handlers() must carry at least one #[CommandHandler] or #[QueryHandler] attribute, otherwise CQRSException is thrown at registration time rather than at dispatch.
• Transactional command handlers. Extend CommandHandlerTransactionService and implement run() instead of process(). The base class wraps the call in Database::startTransaction() / commit() / rollback(), so any exception thrown inside run() rolls the transaction back. Includes a CreateUserCommandHandler example.
• Resetting the registry in tests. Registration holds commands and queries in static state. CQRSRegistration::reset() clears it between test cases, with a setUp() example.

The new material sits right after the existing CQRSRegistration::handlers([...]) example and before the controller usage section.

Verification

Documentation only, no code changes.