Add LiteLLM to acknowledgements section (openai#1240)

## Summary
- Added LiteLLM to the acknowledgements section in README.md
- Recognized LiteLLM as a unified interface for 100+ LLMs, which aligns
with the SDK's provider-agnostic approach

## Test plan
- [x] Verify README renders correctly
- [x] Ensure link to LiteLLM repository is functional

Author: colesmcintosh

README.md

@@ -299,6 +299,7 @@ make format-check # run style checker
 We'd like to acknowledge the excellent work of the open-source community, especially:
 
 -   [Pydantic](https://docs.pydantic.dev/latest/) (data validation) and [PydanticAI](https://ai.pydantic.dev/) (advanced agent framework)
+-   [LiteLLM](https://github.com/BerriAI/litellm) (unified interface for 100+ LLMs)
 -   [MkDocs](https://github.com/squidfunk/mkdocs-material)
 -   [Griffe](https://github.com/mkdocstrings/griffe)
 -   [uv](https://github.com/astral-sh/uv) and [ruff](https://github.com/astral-sh/ruff)