Add comprehensive metrics documentation to README

[bartveneman]

Summary

Added detailed documentation of all CSS analysis metrics to the README, organized by category (Performance, Maintainability, and Complexity). This provides users with clear visibility into what each metric measures and how points are deducted.

Changes

• Added a new "Metrics" section to the README with three subsections:
  • Performance: Documents 7 metrics including imports, empty rules, duplications, file size, comments, and embedded content
  • Maintainability: Documents 7 metrics covering lines of code, selector/declaration counts, and distribution analysis
  • Complexity: Documents 6 metrics for selector complexity, specificity, and ID/important selector ratios
• Each metric includes a descriptive table showing what is tested and the point deduction formula
• Organized metrics in a logical flow that mirrors the package's scoring categories mentioned in the introduction

Details

The documentation provides users with a transparent reference for understanding how the CSS analysis scoring works, making it easier to interpret results and identify areas for CSS improvement.

https://claude.ai/code/session_011qjCuV5Cpr4xAr5H57tGcv

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2086b73) to head (6b0fa90).

Additional details and impacted files

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##              main       #59   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines          149       149           
  Branches        21        21           
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  Hits           149       149           

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