Add comprehensive README.md documentation

[Copilot]

This PR replaces the minimal 2-line README.md with comprehensive documentation that provides users with all the information they need to understand, install, and use the upm-embed Unity extension.

What was added:

Project Overview & Purpose

• Clear explanation of what upm-embed does and why it's useful
• Detailed feature list highlighting key capabilities

Installation Instructions

• Three different installation methods to accommodate different workflows:
  • Direct download and import
  • Git submodule integration
  • UPM Git URL installation (Unity 2019.3+)

Usage Guide

• Step-by-step instructions with clear numbered steps
• Visual examples showing project structure before and after embedding
• Context menu navigation details

Technical Information

• Unity version requirements and compatibility
• Project structure explanation
• Technical details about how the extension works using Unity's PackageManager API

User Support

• Troubleshooting section covering common issues
• Contact information and repository links
• Contributing guidelines for community participation
• License information reference

Additional Features

• Changelog section for version tracking
• Professional formatting with proper markdown structure
• Clear section organization for easy navigation

The README now serves as a complete guide that enables users to quickly understand the project's value proposition and get started with minimal friction.

Fixes #5.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.