Update README.md

[anirudherabelly]

Summary

Fix installation instructions in README.md for developers working on the openai-agents-python repository itself. The original instructions failed when developers tried to install optional dependencies (voice and redis extras) because:

1. For pip: pip install 'openai-agents[voice]' would install from PyPI instead of the local codebase, preventing developers from testing their changes.

2. For uv: uv add 'openai-agents[voice]' would attempt to add the project as its own dependency, which uv prevents with the error: "self-dependencies are not permitted without the --dev or --optional flags."

[anirudherabelly]

@seratch, Please take a look.