Expand README with comprehensive documentation and examples

[dterziev]

Summary

This PR significantly expands the README.md with comprehensive documentation, usage examples, and API reference to improve discoverability and ease of use for new users.

Changes

• Enhanced project description – Updated the opening description to be more detailed and highlight key capabilities (streaming, automatic mapping, fluent API, computed columns, multi-target support)
• Added Features section – Documented core features including lazy streaming, automatic property discovery, fluent mapping API, computed columns, and multi-target framework support
• Added Installation section – Brief guidance on adding the library to a project
• Expanded Usage section with multiple examples:
  • Complete SqlBulkCopy integration example
  • Default mapping (auto-discover all public properties)
  • Expression-based mapping with lambda expressions
  • Delegate-based mapping with explicit names, types, and computed values
  • Mixed mapping styles combining both approaches
• Added API Reference section – Documented EnumerableExtensions and ColumnMappings<T> with method signatures and descriptions
• Added Building, Testing, and Benchmarks sections – Quick reference for developers
• Added License section – MIT license attribution

Notable Details

• Examples use realistic data volumes (10,000 items) and modern async patterns
• Code examples demonstrate both simple and advanced scenarios
• API documentation uses a clear table format for easy scanning
• All examples are self-contained and runnable

https://claude.ai/code/session_01DmmRHcgn5V1ZSmQbKT3Jbh