Add comprehensive README.md with API documentation and examples

[Copilot]

This PR adds a comprehensive README.md file to document the java-jsonpath-api project. The README provides complete documentation for users and developers, including:

What's Added

• Project Overview: Clear description of what the API does and its use cases
• Quick Start Guide: Step-by-step instructions to build and run the application
• Complete API Documentation: Detailed endpoint documentation with request/response formats
• Working Examples: Multiple tested examples showing:
  • Basic JSON queries ($.name, $.age)
  • Array operations ($.users[*].age, $.users[0].name)
  • Complex nested structures ($.store.book[*].title, $.store..price)
• JSONPath Quick Reference: Common JSONPath expressions and their usage
• Docker Instructions: How to build and run with Docker/Docker Compose
• Deployment Guide: DigitalOcean App Platform and other cloud platform instructions
• Development Setup: Project structure and development workflow
• Error Handling: HTTP status codes and error response formats

Technical Improvements

• Updated .gitignore to exclude Maven target/ directory from version control
• Fixed Maven assembly plugin configuration (changed attached to single goal)
• All README examples have been tested against the running application

The documentation is structured to help both new users get started quickly and developers understand the codebase for contributions.

Fixes #1.

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