Add slack alternatives.

replace/slack.md

@@ -16,3 +16,22 @@ All-in-one secure chat app for teams, friends and organisations. Keeps conversat
   - **License**: [Apache License 2.0](https://github.com/vector-im/riot-web/blob/develop/LICENSE)
 {% endinfobox %}
 
+## Mattermost
+
+An Open-Source and self-hosted slack alternative.
+
+{% infobox %}
+  - **Website**: [mattermost.com](https://mattermost.com/)
+  - **Source code**: [Github](https://github.com/mattermost/mattermost-server)
+  - **License**: [Apache and MIT License](https://github.com/mattermost/mattermost-server/blob/master/LICENSE.txt)
+{% endinfobox %}
+
+## Rocket Chat
+
+An Open-Source team collaboration tool that can be self-hosted.
+
+{% infobox %}
+  - **Website**: [rocket.chat](https://rocket.chat)
+  - **Source code**: [Github](https://github.com/RocketChat/Rocket.Chat)
+  - **License**: [MIT](https://github.com/RocketChat/Rocket.Chat/blob/develop/LICENSE)
+{% endinfobox %}
\ No newline at end of file