Add example for Immutables+MapStruct

Nadie ha tomado este issue todavía.

Would be nice to have an example that shows the usage of MapStruct with [Immutables](https://immutables.github.io/).

I was trying to use MapStruct with Immutables and couldn't find a comprehensive example or guide that could walk users through this set up. Would be nice to have this example.

I created a [sample](https://github.com/deepaks829/demo) Spring REST service that uses MapStruct and Immutables to post my [question](https://stackoverflow.com/questions/55888960/mapstruct-does-not-detect-setters-in-builder) on StackOverflow.

If you think this is a good addition, with some help and guidance I am willing to improve this to have it included as an example.

No se mencionan archivos del repositorio ni pruebas. Empieza revisando los ejemplos existentes en mapstruct-examples y el servicio REST de Spring de demostración enlazado; después, compara su configuración de MapStruct e Immutables con las convenciones del proyecto. El trabajo estará terminado cuando se añada un ejemplo completo o una guía que demuestre la integración.

Escrito por el modelo de indexación a partir del texto del issue.