docs(readme): add bundle size badge (#426)

Author: stainless-bot

README.md

@@ -1,6 +1,6 @@
 # Anthropic TypeScript API Library
 
-[![NPM version](https://img.shields.io/npm/v/@anthropic-ai/sdk.svg)](https://npmjs.org/package/@anthropic-ai/sdk)
+[![NPM version](https://img.shields.io/npm/v/@anthropic-ai/sdk.svg)](https://npmjs.org/package/@anthropic-ai/sdk) ![npm bundle size](https://img.shields.io/bundlephobia/minzip/@anthropic-ai/sdk)
 
 This library provides convenient access to the Anthropic REST API from server-side TypeScript or JavaScript.