Added Gitter badge

Author: gitter-badger

README.md

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 OpenSSE: Open Sketch Search Engine [![Build Status](https://travis-ci.org/zddhub/opensse.svg?branch=master)](https://travis-ci.org/zddhub/opensse)
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+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/zddhub/opensse?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 Hello, everybody! 
 
 I'm very interested in [Mathias Eitz's works](http://cybertron.cg.tu-berlin.de/eitz/projects/sbsr/) on SIGGRAPH 2012, so I developed OpenSSE, Just for fun.