Add comment for other annotation processors for the installation

A lot of users are having problems, because they don't quite understand the maven compiler annotationProcessorPaths. @alexis-puska suggested that we extend the installation instructions with something like:

<configuration>
    <source>1.6</source> <!-- or higher, depending on your project -->
    <target>1.6</target> <!-- or higher, depending on your project -->
    <annotationProcessorPaths>
        <path>
            <groupId>org.mapstruct</groupId>
            <artifactId>mapstruct-processor</artifactId>
            <version>${org.mapstruct.version}</version>
        </path>
        <!-- If you use other annotations processor add them here -->
    </annotationProcessorPaths>
</configuration>

I think that this is a good suggestion and would help other developers in issues when MapStruct and Lombok are not working well together due to wrong setup

从 issue 中链接的安装页面开始，检查其中的 Maven 编译器配置。完成的标准是，安装说明解释用于额外注解处理器的 annotationProcessorPaths，并包含提议的配置示例，同时涵盖 MapStruct/Lombok 配置的上下文。