extend use cases

[nsheff]

We got some feedback that the use case in the spec were technical, and not really user-oriented.

Also, see #11

I've done 2 things to address this:

1. I added in a paragraph that describes the primary use cases.
2. I went back through the old user stories document and added in a list of more use-oriented, less technical use cases

I think these provide some useful examples to help give readers an idea of the scope and intent of the standard.

[sveinugu]

A few duplications/grammatical errors that should be fixed. Added a suggestion to add a genome browser use case, but that can be ignored if you think it doesn't fit.

Otherwise, LGTM