Add a README.txt to describe scenarios

I'm starting to forget what these are, and no one else is going to
have a clue.  So let's add a file where we can have more of a
description of each scenario.
Return public types for NewXXX methods

Whilst we want to be able to get an instance of the private types via
the MOCK().NewXXX methods, we want to wrap them in a public type so
that test code can declare variables (e.g. to write utility methods
that return or setup mocks).