docs(readme): add alpha callout

Author: RobertCraigie

README.md

@@ -1,3 +1,9 @@
+> [!IMPORTANT]  
+> We're actively working on a new alpha version that migrates from `node-fetch` to builtin fetch.
+> 
+> Please try it out and let us know if you run into any issues!
+> https://github.com/openai/openai-node/discussions/1239
+
 # OpenAI TypeScript and JavaScript API Library
 
 [![NPM version](https://img.shields.io/npm/v/openai.svg)](https://npmjs.org/package/openai) ![npm bundle size](https://img.shields.io/bundlephobia/minzip/openai) [![JSR Version](https://jsr.io/badges/@openai/openai)](https://jsr.io/@openai/openai)