add link to Matrix channel

Signed-off-by: Daniel Maslowski <info@orangecms.org>

Author: orangecms

content/_index.md

@@ -54,10 +54,12 @@ to your favorite calendar app.
 ### Contact
 
 * [Mailing list](https://groups.google.com/forum/#!forum/linuxboot)
-* [Slack](https://osfw.slack.com/messages/linuxboot) (Join
-  [here](https://slack.osfw.dev))
+* [Matrix (chat)](https://matrix.to/#/#osfw-linuxboot:matrix.org)
+* [Slack (chat)](https://osfw.slack.com/messages/linuxboot) (Join [here](https://slack.osfw.dev))
 * [Twitter](https://twitter.com/LinuxBootOrg)
 
+**NOTE**: Matrix and Slack are not bridged, but we are present in both.
+
 ### Organizations Involved
 
 The following are just a subset of the organizations and people involved.