note distinctive strengths in readme

[lestrrat]

• Sharpen the intro to name the distinctive trio: fluent builder, reusable compiled validators, ahead-of-time validator code generation
• Reframe the code-generation feature around its real advantage — emitting standalone Go validator source, no embedded schemas or runtime compilation
• Add a tasteful "Why this library?" section positioning the build → compile → validate → generate path against single-purpose alternatives, without naming or bashing them

Positioning informed by a review of the current Go JSON Schema landscape (santhosh-tekuri, google/jsonschema-go, kaptinlin, xeipuuv, invopop, omissis): none combine a fluent builder, compiled reusable validators, and ahead-of-time validator code generation.