Add comprehensive GitHub Copilot instructions to repository

[Copilot]

Added detailed GitHub Copilot usage instructions to help contributors leverage Copilot effectively when working with the Net.Code.Csv library.

Changes Made

New COPILOT.md File

Created a comprehensive guide covering:

• Project Overview: Architecture using DataReader pattern and factory methods
• Repository Structure: Clear mapping of key files and their purposes
• Design Patterns: Factory pattern, DataReader pattern, and configuration objects
• Copilot Usage Patterns: Specific prompts and examples for CSV parsing tasks
• Common Scenarios: Large file processing, different CSV formats, dynamic processing
• Testing Patterns: Unit test structure following existing conventions
• Performance Considerations: Memory efficiency and parsing optimization
• Integration Guidelines: Best practices for using Copilot with this codebase

Updated README.md

Added a new "GitHub Copilot Usage" section that references the detailed COPILOT.md file, making it easily discoverable for contributors.

Key Features

The instructions provide repository-specific guidance including:

• Contextual Prompts: Examples like "Parse CSV with custom delimiter and trim whitespace from unquoted fields only"
• Code Examples: Demonstrating the library's factory methods and configuration patterns
• Error Handling: Guidance for malformed CSV handling and edge cases
• Performance Tips: Memory-efficient processing patterns for large files
• Testing Patterns: Aligned with existing test structure and conventions

This will help contributors write better prompts and get more relevant suggestions from GitHub Copilot when working with CSV parsing functionality.

Fixes #5.

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[Copilot]

@jhgbrt 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

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