docs(readme): add Greenkeeper badge

README.md

 # GLPI API Library for Java
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/glpi-project/java-library-glpi.svg)](https://greenkeeper.io/)
+
 ![GLPI banner](https://user-images.githubusercontent.com/29282308/31666160-8ad74b1a-b34b-11e7-839b-043255af4f58.png)
 
 [![License](https://img.shields.io/github/license/glpi-project/java-library-glpi.svg?&label=License)](https://github.com/glpi-project/java-library-glpi/blob/develop/LICENSE.md)