Add comment about xml-apis dependency.
Comment from old Vespa repo git log.
<name>${project.artifactId}</name>
<dependencies>
<dependency>
+ <!-- Newer version than the one in rt.jar, including the ElementTraversal class needed by Xerces (Aug 2015, still valid Sep 2017) -->
<groupId>xml-apis</groupId>
<artifactId>xml-apis</artifactId>
<version>1.4.01</version>
