Add documentation for event custom property builder

[romm]

See:

NotiZ/Classes/Core/Event/AbstractEvent.php

Lines 146 to 153 in af8cb88

	* To use a custom builder, you need to create a class with the same name as
	* your event at which you append `PropertyBuilder`. The method `build` of
	* your builder will then be automatically called when needed.
	*
	* Example:
	*
	* `MyVendor\MyExtension\Domain\Event\MyEvent` -> Event
	* `MyVendor\MyExtension\Domain\Event\MyEventPropertyBuilder` -> Builder