[Bug] Fix explanation of Pydantic datetime issue

[cretz]

Describe the bug

Today the README says to mark datetime as passthrough, but the right answer is to mark it as unrestricted. Fix this in README, and also consider providing snippet from https://github.com/temporalio/samples-python/blob/4303a9b15f4ddc4cd770bc0ba33afef90a25d3ae/pydantic_converter/worker.py#L45-L65.

Also clarify in README that you should pass through non-workflow imports, not just third party.