add openai env example and update README

[VittorioYan]

#746
Show some commonly used litellm configurations

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🚀	This description was created by Ellipsis for commit 9af5ff7

Summary:

Added optional OpenAI environment variables to .env.example and updated README.md for configuration clarity.

Key points:

• Added OPENAI_API_BASE and OPENAI_ORGANIZATION to .env.example for optional OpenAI configurations.
• Updated README.md to include descriptions for OPENAI_API_BASE and OPENAI_ORGANIZATION environment variables.
• Ensured consistency in the environment variables table formatting in README.md.

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Generated with ❤️ by ellipsis.dev

[ellipsis-dev]

👍 Looks good to me! Reviewed everything up to 9af5ff7 in 9 seconds

More details

• Looked at 60 lines of code in 2 files
• Skipped 0 files when reviewing.
• Skipped posting 1 drafted comments based on config settings.

1. README.md:266

• Draft comment:
  In the Environment Variables table, ensure consistent spacing between the 'Description' and 'Type' columns for better readability.
• Reason this comment was not posted:
  Confidence changes required: 50%
  The addition of the OpenAI environment variables in the .env.example file is consistent with the README update. However, the README update has some formatting issues.

Workflow ID: wflow_SZrMRcAvM3P5fgHM

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[wintonzheng]

Thanks!