chore: add contributing guide

[Vadman97]

Note

Adds CONTRIBUTING.md detailing contribution process, Conventional Commits, build/test setup, code structure, and documentation guidelines.

• Documentation:
  • Add CONTRIBUTING.md covering:
    • Issue and PR submission process, including Conventional Commits format and breaking change notation.
    • Build prerequisites, setup, build, and test commands (swift build, swift test).
    • Project structure overview (Sources/, Tests/, ExampleApp/).
    • Guidance on documenting types and methods for consistency.

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