New topic about "Map-to-map cascading"

<map>
    <topicref="a.ditamap" format="ditamap" toc="no"/>
    <topicref="b.ditamap" format="ditamap" audience="developer"/>
    <topicref="c.ditamap" format="ditamap" scope="peer"/>
    <topicref="d.ditamap" format="ditamap" print="no"/>
</map>