Docs: mention Matrix channel, too

CONTRIBUTING.md

@@ -35,8 +35,8 @@ in Europe, but feel free to idle. **DO NOT** ask-and-leave. Keep
 that chat window open because it can easily take a few hours for
 someone to notice a message.
 
-[![Visit our IRC channel](https://kiwiirc.com/buttons/webchat.freenode.net/calamares.png)](https://webchat.freenode.net/?channel=#calamares?nick=guest)
-
+- [![Visit our IRC channel](https://kiwiirc.com/buttons/webchat.freenode.net/calamares.png)](https://webchat.freenode.net/?channel=#calamares?nick=guest)
+- [Join us on Matrix](https://webchat.kde.org/#/room/%23calamares:kde.org)
 
 ## General Guidelines
 

README.md

@@ -12,7 +12,7 @@
 [![GitHub license](https://img.shields.io/github/license/calamares/calamares.svg)](https://github.com/calamares/calamares/blob/calamares/LICENSE)
 
 
-| [Report a Bug](https://github.com/calamares/calamares/issues/new) | [Translate](https://www.transifex.com/projects/p/calamares/) | [Contribute](CONTRIBUTING.md) | [Freenode (IRC): #calamares](https://webchat.freenode.net/?channel=#calamares?nick=guest) | [Wiki](https://github.com/calamares/calamares/wiki) |
+| [Report a Bug](https://github.com/calamares/calamares/issues/new) | [Translate](https://www.transifex.com/projects/p/calamares/) | [Contribute](CONTRIBUTING.md) | [Freenode (IRC): #calamares](https://webchat.freenode.net/?channel=#calamares?nick=guest) | [Matrix](https://webchat.kde.org/#/room/%23calamares:kde.org) | [Wiki](https://github.com/calamares/calamares/wiki) |
 |:--:|:--:|:--:|:--:|:--:|