docs: write full README

[Apetuezekiel]

What

Replaces the stub README with complete library documentation.

Sections:

• Installation
• 60-second quick start (defineConfig + createAccess + diff)
• Full API reference: defineConfig, createAccess, AccessChecker (can/cannot/guard/diff), Overrides, AccessDeniedError, InvalidOverrideError, Limit, ConfigInput/PlanInput schema
• Override examples: grandfathering (grant) and compliance removal (revoke)
• Diff example for upgrade prompt rendering
• Dynamic config section: how literal inference is lost with JSON input and how to use the JSON Schema to validate it
• "What the library does not do": usage tracking, period reset, persistence, remote config, auth, billing, framework bindings — all explicitly enumerated
• Framework glue patterns (React hook, Express middleware) — documentation only, not published
• Comparison to LaunchDarkly and Stripe Entitlements
• Versioning and stability policy (semver, 0.x caveat)

Why

Implements PR 5 from the v1 scope. The README is the primary onboarding surface; without it, the library is unusable regardless of code quality.

How to verify

Read the rendered README on GitHub. Verify:

• All exported symbols are documented
• what the library does not do section explicitly names the items from the brief
• Dynamic config section explains type inference loss
• Versioning section mentions 0.x instability caveat

Out of scope

• CHANGELOG — PR 6
• Version bump — PR 6