docs: create README for goauth

[Copilot]

The repository had an empty README with no documentation. This adds a complete reference covering all three packages.

Changes

• Quick start — end-to-end wiring example (JWT setup → middleware → handler mounting)
• auth package — JWTManager, Middleware/AdminMiddleware, RateLimiter (including trusted-proxy config), and crypto utilities (HashHighEntropyToken, GenerateRandomHex, SecretEncrypter)
• Store interfaces — UserStore, APIKeyStore, PasskeyStore with sentinel error contracts (sql.ErrNoRows, auth.ErrEmailExists)
• handler package — route tables and config structs for AuthHandler, OIDCHandler (PKCE + account-linking flow), APIKeyHandler, and PasskeyHandler (discoverable WebAuthn login)
• smtp package — environment variable reference and LoadConfig/Validate/Send usage
• Security notes — secret length minimums, timing-safe login, PKCE, rate limiting placement, cookie flags, trusted proxy CIDR scoping