Add ReadTheDocs documentation badge to README

Add official ReadTheDocs documentation badge linking to clustrix.readthedocs.io.

The badge provides:
- Visual indication of documentation availability
- Direct link to hosted documentation
- Status indicator for documentation build health

Badge URL: https://clustrix.readthedocs.io/en/latest/
Badge image: https://readthedocs.org/projects/clustrix/badge/?version=latest

Enhances project visibility and accessibility of comprehensive documentation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jeremymanning

README.md

@@ -1,6 +1,7 @@
 # Clustrix
 
 [![Tests](https://github.com/ContextLab/clustrix/actions/workflows/tests.yml/badge.svg)](https://github.com/ContextLab/clustrix/actions/workflows/tests.yml)
+[![Documentation](https://readthedocs.org/projects/clustrix/badge/?version=latest)](https://clustrix.readthedocs.io/en/latest/?badge=latest)
 [![Python 3.8+](https://img.shields.io/badge/python-3.8+-blue.svg)](https://www.python.org/downloads/)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)