Update README.md

Author: alshedivat

README.md

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 [![demo](https://img.shields.io/badge/theme-demo-brightgreen.svg)](https://alshedivat.github.io/al-folio/)
 [![license](https://img.shields.io/github/license/mashape/apistatus.svg?maxAge=2592000)](https://github.com/alshedivat/al-folio/blob/master/LICENSE)
+[![gitter](https://badges.gitter.im/alshedivat/al-folio.svg)](https://gitter.im/alshedivat/al-folio?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 A simple and clean [Jekyll](https://jekyllrb.com/) theme for academics.