Update README with built-in type support, dynamic serialization API a…

…nd Unity info (#133)

* Update README with built-in type support and Unity info

Expanded documentation to list rich built-in type support, clarified serialization APIs and AOT limitations, and added a section on Unity compatibility. These changes improve clarity for users regarding supported types, usage patterns, and platform support.

* Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

* Update README.md

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Co-authored-by: Copilot <[email protected]>