docs: expand README with installation, usage examples, and technical overview

[Copilot]

The README contained only a one-line description and a warning. It lacked any guidance on how to install or use the library.

Installation

Added three installation paths: dotnet add package, Install-Package, and direct <PackageReference>.

Usage examples

Added C# snippets for both directions of the serialization API:

// Deserialize
BurritoSerializationRoot? burrito = BurritoSerializer.Deserialize(json);

// Serialize
string json = BurritoSerializer.Serialize(burrito);

Technical overview (bottom of README)

• Architecture section covering BurritoSerializer (public API) and BurritoContext (source-gen JsonSerializerContext)
• Reference table of all 16 models/enums in ScriptureBurrito.Models
• Documents the active System.Text.Json options: camelCase property naming, null omission, and string enum converters for MetaCategory and MetaNormalization

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