updated explanations on Doxia dependency

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1643694 13f79535-47bb-0310-9956-ffa450edef68
[MSHARED-389] created an IT to show how to implement a report that uses an external tool to generate the report

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1643698 13f79535-47bb-0310-9956-ffa450edef68