docs: #2023 update readme to reflect commands required for developers

[anirudherabelly]

Summary

Fixes 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."

This PR adds developer-specific notes that clarify the correct commands:

• For pip: Use pip install -e '.[voice]' to install in editable mode from the local directory
• For uv: Use uv sync --extra voice to sync optional dependencies without adding the project as a dependency

The changes maintain backward compatibility—end users can still follow the original instructions, while developers now have clear guidance.

Test plan

1. Verified that the existing installation commands still work for end users (they install from PyPI)
2. Tested that uv sync --extra voice works correctly in the repository
3. Verified that pip install -e '.[voice]' would work (editable install from local directory)
4. Confirmed that uv add 'openai-agents[voice]' fails with the expected error message when run inside the repository
5. Ran linting and formatting checks

Issue number

#2023

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[anirudherabelly]

@seratch Could you please take a look?

[seratch]

Thanks for sending this! However, the readme is mainly for users, so we don't have to have these changes. If you're interested in contributing to this repo, you can use make sync and other makefile commands for many local dev operations.