Add example for Immutables+MapStruct

还没有人认领这个 Issue。

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.

未指定任何仓库文件或测试。首先检查 mapstruct-examples 中现有的示例和链接的演示 Spring REST 服务，然后将其 MapStruct 和 Immutables 配置与项目约定进行比较。添加一个展示该集成的完整示例或指南后，即视为完成。

由索引模型根据 Issue 内容生成。