Update README.md

Author: frsyuki

README.md

@@ -8,6 +8,8 @@ MessagePack for Java
 MessagePack v7 (or later) is a faster implementation of the previous version [v06](https://github.com/msgpack/msgpack-java/tree/v06), and
 supports all of the message pack types, including [extension format](https://github.com/msgpack/msgpack/blob/master/spec.md#formats-ext).
 
+[JavaDoc is available at javadoc.io](https://www.javadoc.io/doc/org.msgpack/msgpack-core).
+
 ## Quick Start
 
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