Document backing store

Is your feature request related to a problem? Please describe.
When I first used this SDK, I modified objects that were returned but just for the needs of my script, with no intention of pushing back to the API the modifications. It didn't work properly and I had a couple of issues that I didn't understand at first, until I noticed that this SDK uses a notion of "backing store". I didn't dig deeper what it was but now I understand thanks to this description in the Java Graph SDK for the "backing store" and "dirty" mechanisms which I believe are the same as in this Python SDK.

Describe the solution you'd like
If it's indeed the same thing, please raise awareness about it in the README (or elsewhere) to reduce confusion

Describe alternatives you've considered
Perhaps I could have spent more time and managed to find the article myself, or this https://learn.microsoft.com/en-us/openapi/kiota/models#backing-store

Review the README and the linked Microsoft Graph Java and Kiota backing-store descriptions. Confirm how the Python SDK's backing store and dirty mechanisms behave, then document them in the README or another appropriate location so users understand when returned-object changes are tracked or sent back to the API.