Document request timeouts — default client has none #2

From the SDK review 2026-09-15.

Evidence: each API class builds new Client() with no options (lib/Api/AccountApi.php:94), so Guzzle waits indefinitely on a hanging server. The fix is one constructor argument — it just needs documenting.

Fix: add a timeout example to the README (new Client(['timeout' => 10]) passed into the API class) plus one sentence discouraging production use without it. (Same gap in the Go/Python/JS SDKs, with issues there.)