The Decoration XSD is located here.
Your favorite IDE probably supports XSD schema's for .xml files. You need to specify the following:
<book xmlns="http://maven.apache.org/BOOK/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/BOOK/1.0.0 http://maven.apache.org/xsd/book-1.0.0.xsd"> ... </book>