Add Gitter badge

Author: gitter-badger

README.md

@@ -1,6 +1,6 @@
 # FIXME Lesson title
 
-[![Create a Slack Account with us](https://img.shields.io/badge/Create_Slack_Account-The_Carpentries-071159.svg)](https://swc-slack-invite.herokuapp.com/)
+[![Create a Slack Account with us](https://img.shields.io/badge/Create_Slack_Account-The_Carpentries-071159.svg)](https://swc-slack-invite.herokuapp.com/) [![Join the chat at https://gitter.im/ESMValGroup/Tutorial](https://badges.gitter.im/ESMValGroup/Tutorial.svg)](https://gitter.im/ESMValGroup/Tutorial?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 This repository generates the corresponding lesson website from [The Carpentries](https://carpentries.org/) repertoire of lessons.