Add warning callout: SDK is for testing/light usage only

Adds a prominent warning to the README advising that this SDK is
intended for testing and light usage only, and recommending direct
HTTP API calls for production traffic at scale.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Bearclaw

README.md

@@ -16,6 +16,9 @@ The AssemblyAI JavaScript SDK provides an easy-to-use interface for interacting
 which supports async and streaming transcription.
 It is written primarily for Node.js in TypeScript with all types exported, but also [compatible with other runtimes](./docs/compat.md).
 
+> **⚠️ WARNING**
+> This SDK is intended for **testing and light usage only**. It is not recommended for use at scale or with production traffic. For best results, we recommend calling the AssemblyAI API directly via HTTP request. See our [official documentation](https://www.assemblyai.com/docs) for more information, including HTTP code examples.
+
 ## Documentation
 
 Visit the [AssemblyAI documentation](https://www.assemblyai.com/docs) for step-by-step instructions and a lot more details about our AI models and API.