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

@@ -18,6 +18,9 @@
 
 With a single API call, get access to AI models built on the latest AI breakthroughs to transcribe and understand audio and speech data securely at large scale.
 
+> **⚠️ 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.
+
 # Overview
 
 - [AssemblyAI's Python SDK](#assemblyais-python-sdk)