Add example for Immutables+MapStruct

まだ誰も着手していません。

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.

リポジトリのファイルやテストは指定されていません。まず mapstruct-examples にある既存の例と、リンクされているデモ用 Spring REST サービスを確認し、その MapStruct と Immutables の設定をプロジェクトの規約と比較してください。統合を実証する包括的な例またはガイドを追加すれば完了です。

索引モデルが issue の本文から書いたものです。