Adds an attribute node to the receiver.
Original signature is '- (void)addAttribute:(NSXMLNode *)anAttribute'
Available in Mac OS X v10.4 and later.


- anAttribute (NSXMLNode)
- An XML node object representing an attribute. If the receiver already has an attribute with the same name, anAttribute is not added.

- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)