Add README.md

[kudima03]

Summary

• Adds README.md with a one-line tagline describing the package's role in the Pure ecosystem
• Includes CI badges for all three workflows (build-and-test.yml, Benchmarks.yml, publish-nuget.yml) plus NuGet and license badges
• Documents the full DeterminedHash constructor overload table covering all 15 input types
• Lists both Pure dependencies (Pure.HashCodes.Abstractions and Pure.Primitives.Abstractions) with version-pinned GitHub links and one-line descriptions
• Provides a brief, realistic usage snippet for single-primitive and compound hashing

Closes #71

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Pure.HashCodes	98%	50%	118	➖
Summary	98% (573 / 583)	50% (1 / 2)	118	➖

Minimum allowed line rate is 98%