usage scenarios document

 What have been done since last f2f?

Based on the original proposal from Hugo:

1. Structure change
	moved two use cases to the front.
2. Link management
	specref to sceniors are extracted and reverse links are created for
those sceniors that are cited.


issues
1. Not all sceniors are cited in use cases
	- cite more sceniors in use cases
	- write new use cases
2. Use cases maybe too complicated
	- write some simple use cases

3. Not consistent term, sceniors and use cases are interchanged

4. Naming convention of sceniors is not friendly for inserting new sceniors

5. usage sceniors are not maintained.
	- go through and remove unnecessary ones
	- add new ones



additional to-dos

1. provide scenior citation table for each use case