ISSUE-77: Reference threat work in wsc-usecases

Concensus at the Dublin meeting was to reference our threat tree work. I'm 
going to take another stab at a proposal, against the current text of the 
draft. 

In the threats section of the draft:
http://www.w3.org/2006/WSC/drafts/note/#threats

I propose to change the second sentence to include a reference to the 
threat tree work in the wiki, as such:

"Threats which are in-scope for this Working Group are further discussed 
below, as well as in the WSC wiki [ref 
http://www.w3.org/2006/WSC/wiki/ThreatTrees]."