These files are copied from the following locations:

 * docbookxi.rng: http://docbook.org/xml/5.1CR1/rng/docbookxi.rng
 * extensions.rng: https://raw.githubusercontent.com/openstack/clouddocs-maven-plugin/master/src/main/resources/rng/extensions.rng
 * rackbook.rng: https://raw.githubusercontent.com/openstack/clouddocs-maven-plugin/master/src/main/resources/rng/rackbook.rng
 * wadl.rng: https://raw.githubusercontent.com/openstack/clouddocs-maven-plugin/master/src/main/resources/rng/wadl.rng
 * wadl.xsd: http://docs.rackspace.com/rackbook/wadl.xsd
 * xml.xsd: http://docs.rackspace.com/rackbook/xml.xsd
