Enhance README for Clarity and Comprehensive Guidance

[cqiao64]

Changes Introduced:

• Refined the Introduction to better highlight Validity's value for web developers and designers.
• Expanded the Features section to include instant validation, error reporting, and custom validation service.
• Included a detailed Chrome Extension Installation Guide.
• Provided an Overview of the W3C Markup Validator Installation process.
• Simplified the How to Contribute section to encourage community contributions.
• Added direct links to GitHub Issues and the Chrome Web Store for the extension.
• Updated the License section to reflect the Apache License, Version 2.0, with a link to the full license text.

Rationale:
These updates aim to improve documentation clarity, making it easier for users and potential contributors to understand and engage with the project.