docs: enhance README with detailed package usage and configuration examples

[j-d-ha]

🚀 Pull Request

📋 Summary

This PR significantly enhances the README documentation to provide comprehensive guidance on using both package options (runtime and source generator), including detailed installation instructions, configuration examples, and a decision guide to help users choose the right approach for their needs.

Key Changes:

• Added package comparison table with NuGet badges and download statistics for both packages
• Expanded installation section with detailed examples for both the source generator and runtime package options
• Added MSBuild configuration examples showing how to configure the source generator with properties like SourceGeneratorToolsInclude and SourceGeneratorToolsUsePublicModifier
• Created "When to Use Which Approach" section with a comprehensive comparison table covering deployment, dependencies, use cases, and other decision factors
• Updated gitignore to exclude .DS_Store files

Documentation Improvements:

• Clear distinction between the two package options (Generator vs Runtime)
• Proper .csproj examples with correct OutputItemType and ReferenceOutputAssembly attributes for the generator package
• Explanation of how CompilerVisibleProperty declarations work with the generator's .props file
• Decision matrix to help users choose between source generator and runtime package

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✅ Checklist

• My changes build cleanly
• I've added/updated relevant tests (N/A - documentation only)
• I've added/updated documentation or README
• I've followed the coding style for this project
• I've tested the changes locally (if applicable)

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🧪 Related Issues or PRs

This builds on the recent MSBuild property support added in #9 by documenting how to use the configuration options.

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💬 Notes for Reviewers

This is primarily a documentation enhancement PR. The README now provides:

1. Clearer package positioning - Users can immediately see there are two packages and understand their purposes
2. Actionable installation guides - Copy-paste ready .csproj examples for both packages
3. Configuration documentation - Shows how to use the new MSBuild properties from feat: Add MSBuild property support for configurable source generation #9
4. Decision guidance - Helps users make informed choices about which package to use

The changes make the project more accessible to new users while providing comprehensive guidance for both common use cases (building source generators vs using utilities in applications).

Please verify:

• The NuGet package names and versions are correct
• The .csproj examples follow best practices
• The comparison table accurately represents the trade-offs between the two approaches