Internal-only API reference for 'contributing' docs

[sadielbartholomew]

Only the public API is generally documented in the API reference, but for developers, those building libraries using cf-python, and anyone else potentially keen to know more about what goes on under-the-hood, without having to search through the codebase, it could be useful to have some developer documentation containing the API reference for internal objects. A sensible place to house that would be in some hierarchy under the existing 'Contributing' section.

See also #210 (comment) where a reference for the mixin classes is given as an example that could be useful.