User stories for git-extract

@charlierudolph @allewun adding user stories that describe end-user value
We tell users they can only run this on feature branches, but the reason why isn't exactly apparent. 

The actual reason is because we determine the extractable commits by finding all the commits in the current branch that are not in the main branch. I would like to update git-extract so it errors if the current branch has no extractable commits. I believe that is easier to understand (and fixes the current poor user experience if trying to extract commits from a feature branch that is even with main).

It also allows you extract commits from non-feature branches (if they have commits not in main). The fact that we were not allowing extracting from non feature branches was an oddity to me.