Improve CONTRIBUTING.md clarity around generated code edits

[yuka-with-data]

Hi Team,

I had a brief discussion with Ahmed and learned that direct edits to SDK code are generally discouraged to avoid conflicts with generated code from the Steinless SDK tool. I also noticed this is partially mentioned in CONTRIBUTING.md, but it may not be immediately clear to new contributors.

It might be helpful to make this more explicit in the contribution guidelines and highlight where contributions are preferred.

For example, a short note under "Modifying/Adding Code" like:

⚠️Most of this SDK is generated using the Stainless SDK tool. 
Manual edits to generated code are discouraged, as they may conflict with future generations. 
Contributions are recommended in:
- src/onebusaway/lib/ and examples/
- documentation
- (and other areas specified by maintainers)

This could help set clear expectations for contributions.
Let me know what you think.