Add mattermost chat/badge

README.md

@@ -1,7 +1,7 @@
 # matterbridge
 Click on one of the badges below to join the chat   
 
-[![Gitter](https://img.shields.io/gitter/room/nwjs/nw.js.svg)](https://gitter.im/42wim/matterbridge) [![Join the IRC chat at https://webchat.freenode.net/?channels=matterbridgechat](https://img.shields.io/badge/IRC-matterbridgechat-green.svg)](https://webchat.freenode.net/?channels=matterbridgechat) [![Discord](https://img.shields.io/badge/discord-matterbridge-green.svg)](https://discord.gg/AkKPtrQ) [![Matrix](https://img.shields.io/badge/matrix-matterbridge-green.svg)](https://riot.im/app/#/room/#matterbridge:matrix.org) [![Slack](https://img.shields.io/badge/slack-matterbridgechat-green.svg)](https://join.slack.com/matterbridgechat/shared_invite/MjEwODMxNjU1NDMwLTE0OTk2MTU3NTMtMzZkZmRiNDZhOA)
+[![Gitter](https://img.shields.io/gitter/room/nwjs/nw.js.svg)](https://gitter.im/42wim/matterbridge) [![Join the IRC chat at https://webchat.freenode.net/?channels=matterbridgechat](https://img.shields.io/badge/IRC-matterbridgechat-green.svg)](https://webchat.freenode.net/?channels=matterbridgechat) [![Discord](https://img.shields.io/badge/discord-matterbridge-green.svg)](https://discord.gg/AkKPtrQ) [![Matrix](https://img.shields.io/badge/matrix-matterbridge-green.svg)](https://riot.im/app/#/room/#matterbridge:matrix.org) [![Slack](https://img.shields.io/badge/slack-matterbridgechat-green.svg)](https://join.slack.com/matterbridgechat/shared_invite/MjEwODMxNjU1NDMwLTE0OTk2MTU3NTMtMzZkZmRiNDZhOA) [![Mattermost](https://img.shields.io/badge/slack-matterbridge-green.svg)](https://framateam.org/signup_user_complete/?id=tfqm33ggop8x3qgu4boeieta6e)
 
 ![matterbridge.gif](https://s15.postimg.org/qpjhp6y3f/matterbridge.gif)