Move USECASES to usecases document, add link to the spec

[mlagally]

The repo contains a directory called USECASES that contain a description of some use cases.

Let's consolidate our document references:

• Please move the use cases from the USECASES subdirectory to the use cases document.

• Please also add a reference to the use cases document and a brief description to the introduction section of the discovery spec.

[mmccool]

This is just cleanup, the content has been moved already, the old content in the Discovery repo just needs to be deleted. I will double-check and take care of this...