docs: document free-tier plan limits (3 segments/audiences) and handling strategy

[asielcabrera]

Description

The Resend free-tier plan limits to 3 audience segments. This causes POST /audiences and POST /broadcasts to fail with HTTP 400 when the limit is reached.

Error

{"statusCode":400,"message":"Your plan includes 3 segments. Upgrade to add more.","name":"validation_error"}

Suggested improvements

1. Document the limit in the README or DocC
2. Add a helper method to check remaining segment quota
3. Provide a better error message or context when this error is thrown
4. Consider a plan-aware approach that warns before hitting limits