feat(devtools): add link to mattermost channel (Jigsaw-Code#1182)

* feat(devtools): add link to mattermost channel

* Update README.md

README.md

@@ -1,12 +1,12 @@
 # Outline Server
 
-![Build and Test](https://github.com/Jigsaw-Code/outline-server/actions/workflows/build_and_test_debug.yml/badge.svg?branch=master)
+![Build and Test](https://github.com/Jigsaw-Code/outline-server/actions/workflows/build_and_test_debug.yml/badge.svg?branch=master) [![Mattermost](https://badgen.net/badge/Mattermost/Outline%20Community/blue)](https://community.internetfreedomfestival.org/community/channels/outline-community) [![Reddit](https://badgen.net/badge/Reddit/r%2Foutlinevpn/orange)](https://www.reddit.com/r/outlinevpn/)
 
 This repository has all the code needed to create and manage Outline servers on
 DigitalOcean. An Outline server runs instances of Shadowsocks proxies and
 provides an API used by the Outline Manager application.
 
-Go to https://getoutline.org for ready-to-use versions of the software.
+Go to https://getoutline.org for ready-to-use versions of the software. **To join our Outline Community, [sign up for the IFF Mattermost](https://internetfreedomfestival.org/wiki/index.php/IFF_Mattermost).**
 
 ## Components