README: illustrate architecture with diagram

[achille-roussel]

This issue is a follow-up to the feedback we got on the Python SDK, in particular:

The other thing I could imagine would be a simple diagram (e.g. Draw.io) showing how the code is in my file, but "this section A runs on your machine", and then "this section B runs on Dispatch servers, possibly across multiple servers, even after failures".

A picture is worth a thousand words, adding a diagram will go a very long way in helping readers form mental models about how they can leverage the product.