maven xml plugin stuck with xsd, but docbook moved to relax

[duncdrum]

Where is the problem

xml-maven-plugin doesn't support relaxNG validation.

What is the problem

Sticking with XSD is suboptimal since relax is the normative format for docbook 5.0 an up. So lets find a way to use relaxNG schema validation.

see #151

On a side note: build failures because of validation errors seem pretty drastic

[adamretter]

It would be fairly trivial to add RelaxNG support to the xml-maven-plugin: https://github.com/mojohaus/xml-maven-plugin/tree/master/src/main/java/org/codehaus/mojo/xml

However, I don't really see an immediate use-case for this.

[duncdrum]

Our prayers have been heard 🎉 relaxng support