[XML-SIG] XML HOWTO updated

[Andrew Kuchling]

I've updated the XML HOWTO to match the current state of PyXML,
updating for SAX2, filling out the DOM section, and adding sections on
XPath and xml.marshal.

      http://pyxml.sourceforge.net/topics/howto/xml-howto.html

Please offer comments, bug reports, and suggestions for additional
topics that should be covered.

--amk