Update README

[danarnold]

Update README.md:

• improve markdown header consistency
• improve markdown formatting
• improve grammar and wording in a few places
• add instructions for running the tests
• add contributing instructions

[Copilot]

Pull Request Overview

This PR updates the README.md file to improve its overall structure and documentation quality. The changes focus on standardizing markdown formatting, improving grammar and readability, and adding helpful documentation for developers.

Key changes include:

• Standardized markdown headers from mixed formats to consistent ## style
• Added comprehensive test running instructions including support for multiple Gemfile configurations
• Added detailed contributing guidelines with step-by-step workflow instructions

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