Add style guide / design tips / other notes document

[jackfirth]

Tentative name: "The Mock Handbook". General idea is that it's an unordered collection of "best practices" like "don't check arguments in mock behaviors" or "don't check mocks if their returned results prove they've been called". Unlike the reference, this doesn't go into technical details about specific procedures and forms. Unlike the guide, this isn't an ordered series of prose sections meant to teach the library's general concepts. This is a good place for corner case advice and "when should I use X vs Y in situations like Z" advice.

[jackfirth]

Another topic: using parameters with mocks to test with a broader scope than define/mock.