'Add a single note annotation to the command.
Arguments:
msg   -- A human readable message.
ntype -- One of: \'info\', \'warning\', \'error\''

def add_note(self, msg='', ntype='info'):

    xml = ET.Element(('{%s}note' % self.namespace))
    xml.attrib['type'] = ntype
    xml.text = msg
    self.xml.append(xml)
