Docs: mention Matrix consistently

CONTRIBUTING.md

@@ -34,6 +34,7 @@ Regular Calamares development chit-chat happens on old-school IRC
 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.
+The Matrix room is relatively new.
 
 - [![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)

README.md

@@ -59,6 +59,7 @@ problems or a new feature to discuss.
 
 Regular Calamares development chit-chat happens on old-school IRC
 (no registration required). Responsiveness is best during the day
-in Europe, but feel free to idle.
+in Europe, but feel free to idle. The Matrix room is relatively new.
 
-[![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)