docs: revise readme docs about nested params

[stainless-app]

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EntelligenceAI PR Summary

A new section on Nested params has been introduced in the README.md to illustrate how to use TypedDict effectively.

[entelligence-ai-pr-reviews]

Walkthrough

Added a section on Nested params in README.md to demonstrate usage with TypedDict.

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LGTM 👍