Add example for Immutables+MapStruct

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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.

Aucun fichier du dépôt ni aucun test n’est mentionné. Commencez par examiner les exemples existants dans mapstruct-examples ainsi que le service REST Spring de démonstration lié, puis comparez sa configuration MapStruct et Immutables avec les conventions du projet. Le travail est considéré comme terminé lorsqu’un exemple complet ou un guide démontrant l’intégration est ajouté.

Rédigé par le modèle d'indexation à partir du texte de l'issue.