Add developer documentation

[redshiftzero]

To make issues like freedomofpress/securedrop-sdk#16 easier for developers, we should add some documentation of exactly what format is expected for the requests and responses (one can figure out from the examples / code but worth writing out explicitly). For example, these docs should describe the valid fields for a request (path_query, body, headers, method). This can go in the regular SecureDrop developer documentation, and be linked to in the README in this repository.

[sssoleileraaa]

[Backlog pruning, 5/16] This is still relevant and as part of this research into how it currently works, one could develop a proposal for interface improvement between the SDK and proxy.