add mattermost (#1291)

README.md

@@ -647,6 +647,7 @@ Any comments, suggestions? [Let us know!](https://github.com/jaywcjlove/awesome-
 * [Rambox](http://rambox.pro/) - Messaging and emailing app that combines common web applications into one. [![Open-Source Software][OSS Icon]](https://github.com/saenzramiro/rambox) ![Freeware][Freeware Icon]
 * [Element](https://element.io/) - Create, share communicate. Chat and call securely. [![Open-Source Software][OSS Icon] ![Freeware][Freeware Icon]](https://github.com/vector-im)
 * [Presentify](https://presentify.compzets.com/) - A mac app to draw on your screen while on calls, highlight your cursor, and more. ![App Store][app-store Icon]
+* [Mattermost](https://mattermost.com/download/) - Mattermost is an open source platform for secure collaboration across the entire software development lifecycle. [![Open-Source Software][OSS Icon] ![Freeware][Freeware Icon]](https://github.com/mattermost/mattermost)
 * [Signal Desktop](https://signal.org/download/) - Fast, simple, secure. Privacy that fits in your pocket. [![Open-Source Software][OSS Icon]](https://github.com/signalapp/Signal-Desktop)
 * [Skype](https://www.skype.com/) - Cross-platform application that provides video chat and voice call services. Users can exchange images, text, video and any other digital documents.
 * [Shift](https://tryshift.com/) - De-clutter your desktop workspace and streamline your accounts, apps, and workflows.