README: add example of how *not* using Dispatch is a problem

[achille-roussel]

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

The other thing I would try to do would be starting with an example of how not using Dispatch would be problematic and error prone, and explain "this would raise an error, the program would die, or you would have to write 50 extra lines of code to handle retries". And then, at that point, when I see what is the obvious problem, introduce Dispatch to the rescue.

We got this suggestion multiple times, since it can be difficult for someone first exposed to Dispatch to grasp the full breadth of value they can get from the product, putting it in opposition to the pain points that exist today can go a long way in to bridge the gap.