Documenting release license

In package.xml 1.0, the license tag only contained the name of the license. In package.xml 2.0, there are two optional attributes, "uri" and "filesource". "uri" contains a uri that identifies the text of a license, such as "http://www.php.net/license" for the PHP License. "filesource" can be used to specify a LICENSE file within an actual package that contains the text of the license.