The Decoration XSD is located here.
Your favorite IDE probably supports XSD schema's for .xml files. You need to specify the following:
<document xmlns="http://maven.apache.org/DOCUMENT/1.0.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/DOCUMENT/1.0.1 http://maven.apache.org/xsd/document-1.0.1.xsd" outputName="..."> ... </document>