Added Gitter badge

Author: gitter-badger

README.md

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 # About QGIS
 
+[![Join the chat at https://gitter.im/qgis/QGIS](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/qgis/QGIS?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 [![Build Status](https://travis-ci.org/qgis/QGIS.svg?branch=master)](https://travis-ci.org/qgis/QGIS)
 
 QGIS is an Open Source Geographic Information System. The project was born in