ISSUE-203 (securityconsiderations): Update Security Considerations [wsc-xit]

ISSUE-203 (securityconsiderations): Update Security Considerations [wsc-xit]

http://www.w3.org/2006/WSC/track/issues/

Raised by: Thomas Roessler
On product: wsc-xit

The security considerations section is very rudimentary. We'll want to make it more extensive before going to Last Call.

Specific questions that I would like us to cover:

- where do we currently rely on specific user behavior to guarantee security?
- where are we making protocol assumptions?
- where are we giving choices that are more or less secure?