update README

README.md

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 [![Maven Central](https://img.shields.io/maven-central/v/io.milvus/milvus-sdk-java.svg)](https://search.maven.org/artifact/io.milvus/milvus-sdk-java/)
 
-Java SDK for Milvus distributed high-performance vector search engine. 
+Java SDK for [Milvus](https://github.com/milvus-io/milvus) distributed high-performance vector search engine. 
 If you want to contribute to this repo, please read our [contribution guidelines](https://github.com/milvus-io/milvus/blob/master/CONTRIBUTING.md).
 
 ## Getting started