exception type count in README is inaccurate (says 11, actually 15)

[purahan]

Summary

The README states "11 specific exception types" in several places, but
errors.py currently defines 15 exception classes (16 including the base
LinguaError), including newer ones like ParameterValidationError,
CircularDependencyError, and RecoveryExhaustedError.

Task

Update the count everywhere it appears in README.md:

• Key Features bullet ("Rich error handling")
• Error Handling section intro

Why This Is a Good First Issue

No code changes, verify the count and update docs. Mirrors PR #7's
approach exactly.