docs: clarify global and quote rate limiters

[silentgeckoaudit3801]

Closes #119.

This updates the README rate-limiting section after maintainer confirmation in the issue.

It documents:

• the global mutating-request limiter and its DEFAULT_MAX = 30 / DEFAULT_WINDOW_MS = 60_000 defaults from src/middleware/rateLimiter.ts
• that src/app.ts excludes /api/v1/quote from the global limiter via skipPaths
• the quote-specific rateLimiter({ max: 10, windowMs: 60_000 }) instance
• the 429 / RATE_LIMITED response and client backoff guidance

Validation: static source/API review only; no project dependencies or local toolchains were run from this environment.

[silentgeckoaudit3801]

Thanks for merging this. Is there an official GrantFox / Maybe Rewarded claim or payment step I should follow for the merged issue #119?