Add badge for javadoc.io

Author: cboehme

README.md

@@ -8,7 +8,7 @@ Originally, Metafacture was developed as part of the [Culturegraph](http://cultu
 
 # Build
 
-[![Build Status](https://secure.travis-ci.org/culturegraph/metafacture-core.svg?branch=master)](https://travis-ci.org/culturegraph/metafacture-core/) [![Dependency Status](https://www.versioneye.com/user/projects/5592673839656100200000d7/badge.svg)](https://www.versioneye.com/user/projects/5592673839656100200000d7) [![Maven Central](https://img.shields.io/maven-central/v/org.culturegraph/metafacture-core.svg)]()
+[![Build Status](https://secure.travis-ci.org/culturegraph/metafacture-core.svg?branch=master)](https://travis-ci.org/culturegraph/metafacture-core/) [![Dependency Status](https://www.versioneye.com/user/projects/5592673839656100200000d7/badge.svg)](https://www.versioneye.com/user/projects/5592673839656100200000d7) [![Maven Central](https://img.shields.io/maven-central/v/org.culturegraph/metafacture-core.svg)]() [![Javadocs](http://www.javadoc.io/badge/org.culturegraph/metafacture-core.svg?color=blue)](http://www.javadoc.io/doc/org.culturegraph/metafacture-core)
 
 See the [.travis.yml](./.travis.yml) file for details on the CI config used by Travis.