Add some Use Case documentation

Include:
* Sample/Template
* Instructions
* The beginning of the CreateNewThing Use Case
Start to fill out the CreateNewThing use case

I've decided to have a top-level `use_cases` directory. This directory will
hold use cases written in Markdown with file names likes 'SomeUseCase.md'.
These use cases will be implemented by Interactors with source code in this
same top-level directory. Interactor filenames will be in the format
'some_use_case.rb'. That sample Interactors' specs will reside in
'spec/use_cases/some_use_case_spec.rb'

Also, refine the Template and Instructions mark-up as you go.