Add README.md documentation for learning microservices project

[Copilot]

Added a simple and comprehensive README.md file to document the learning microservices project.

The README includes:

• Project Overview: Clear description of the Spring Boot microservices learning project demonstrating an e-commerce system architecture
• Architecture: Documentation of the three microservices:
  • Product Service (manages product catalog with MongoDB)
  • Order Service (handles order processing with MySQL)
  • Inventory Service (tracks product inventory with MySQL)
• Technology Stack: Complete list of technologies used including Java 21, Spring Boot 3.5.5, Maven, and database technologies
• Build Instructions: Simple commands for building and testing the project
• Services Reference: Quick reference table showing ports and databases for each service

This documentation will help developers quickly understand the project structure and get started with the codebase.

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