ACTION-243 Propose link from note to threat trees

I propose changing the introductory text to the use-cases section 
(currently 6) to: 

We distinguish a number of properties in the basic use cases that we 
address. We will be looking towards adding attack information as well, 
potentially in the form of threat trees [ref 
http://www.w3.org/2006/WSC/wiki/ThreatTrees]. 



Tracker is hosed Issue wise this morning. I'll turn this into an issue 
when it's working properly again.