Update chatml.md (openai#244)

Author: gdb

chatml.md

@@ -1,3 +1,9 @@
+(This document is a preview of the underlying format consumed by
+ChatGPT models. As a developer, you can use our [higher-level
+API](https://platform.openai.com/docs/guides/chat) and won't need to
+iteract directly with this format today — but expect to have the
+option in the future!)
+
 Traditionally, GPT models consumed unstructured text. ChatGPT models
 instead expect a structured format, called Chat Markup Language
 (ChatML for short).