write a guide "how to implement a client"

Right now our only official client libraries are in JS and Python. Until we have clients in a lot of other languages, we should document the best practices that a Kinto client library should support (offline-first) and techniques for implementing them.

Start by comparing the existing official JavaScript and Python client libraries to identify the client behaviors and offline-first techniques they support. Use that comparison to define the guide's scope, then document the recommended practices and implementation techniques for future Kinto clients; the work is done when the guide covers those recommendations clearly.

Written by the indexing model from the issue text.