Produce a Developer guide for OpenADAS

[mattngc]

A number of issues have been raised around what is good practice and the required code standards in this module, see the discussions in #16, #20, #21, #22 for examples. These issues are arising because new collaborators are starting to implement and contribute their own rates, but the standards and structure of the module is not adequately defined in the documentation.

@CnlPepper has already developed many of the required standards when he refactored the code last year. But we need a more prominent documentation page that explains the standards and how new rates should be implemented.