Update README.md

[blake41]

I noticed you wrote both the openai and anthropic clients, and realized that the open-ai client is very similar to the anthropic client but the anthropic readme doesn't document all the functionality. Figured I would add this to the readme to let others know this functionality exists on the anthropic client as well.

All Submissions:

• [ X] Have you followed the guidelines in our Contributing document?
• [ X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ X] Have you added an explanation of what your changes do and why you'd like us to include them?