Add comprehensive Performance Benchmarks section to README

[Copilot]

This PR adds a comprehensive "Performance Benchmarks" section to the README to help users understand the performance characteristics of the airports gem and make informed decisions about usage in production applications.

What's Added

The new section includes:

📊 Standardized Methodology

• Clear environment specifications (hardware, OS, Ruby versions)
• Defined test scenarios (cold start, warm queries, concurrent access, memory profiling)
• Reproducible benchmarking guidelines

📈 Placeholder Tables for Key Metrics

• Startup Performance: Initial load time, JSON parsing, object instantiation
• Query Performance: Operations per second for all main methods (find_by_iata_code, find_by_icao_code, etc.)
• Memory Usage: Initial, peak, and sustained memory consumption patterns
• Version Comparisons: Framework for tracking performance across gem versions

🎯 Contextual Interpretation Guidelines

Performance guidance segmented by application scale:

• Small applications (< 1,000 queries/day)
• Medium applications (1,000 - 100,000 queries/day)
• High-traffic applications (> 100,000 queries/day)

⚠️ Important Caveats and Limitations

• Dataset size considerations (currently 6,076 airports, ~1.4MB)
• Ruby implementation differences (MRI, JRuby, TruffleRuby)
• Hardware dependency notes
• Search algorithm limitations (linear search for some operations)

🤝 Community Contribution Framework

• Detailed guidelines for submitting benchmarks
• Example benchmark script structure using Ruby's Benchmark and MemoryProfiler
• Requirements for reproducibility and documentation

Why This Matters

The airports gem is a data lookup library that loads 6,076 airports into memory for fast querying. Users need to understand:

• Initial startup costs vs. ongoing query performance
• Memory requirements for their application scale
• When the library's approach (memory vs. speed trade-off) fits their needs

Current State

Since no official benchmarks exist yet, all performance tables contain "TBD" placeholders. This creates a clear framework for future performance testing while being transparent about the current state.

The section is positioned between "Usage" and "Development" in the README to maintain logical flow for users evaluating the library.

Technical Details

• Added /vendor/bundle/ to .gitignore to prevent bundler artifacts from being committed
• All existing tests continue to pass
• Linting passes with no issues
• No changes to actual library code - purely documentation enhancement

This establishes the foundation for comprehensive performance evaluation while providing immediate value through standardized methodology and contribution guidelines.

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