docs: update readme

Author: tomsun28

README.md

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+**The [New Java SDK](https://github.com/THUDM/z-ai-sdk-java) for Z.AI and ZhipuAI has released: [z-ai-sdk-java](https://github.com/THUDM/z-ai-sdk-java)!** It is recommended to use this SDK for better, faster, and long-term support.
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 # ZhipuAI Open Platform Java SDK
 
 [![Maven Central](https://img.shields.io/maven-central/v/cn.bigmodel.openapi/oapi-java-sdk.svg)](https://search.maven.org/artifact/cn.bigmodel.openapi/oapi-java-sdk)

README_CN.md

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+Z.AI 和 智谱AI 的 [全新 Java SDK](https://github.com/THUDM/z-ai-sdk-java) 已经发布：[z-ai-sdk-java](https://github.com/THUDM/z-ai-sdk-java)！推荐使用此 SDK，以获得更好、更快的长期支持。
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 # 智谱AI开放平台 Java SDK
 
 [![Maven Central](https://img.shields.io/maven-central/v/cn.bigmodel.openapi/oapi-java-sdk.svg)](https://search.maven.org/artifact/cn.bigmodel.openapi/oapi-java-sdk)