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

Start at the installation page linked in the issue and review its Maven compiler setup. Done means the installation instructions explain annotationProcessorPaths for additional annotation processors and include the proposed configuration example, with the MapStruct/Lombok setup context covered.