docs: add matrix channel information to normal and github READMEs

(cherry picked from commit 3c19ca90327d92be69cdce482e66991147db9083)

README

@@ -109,7 +109,8 @@ To participate in the mailing list, subscribe at the following web address:
          https://lists.kamailio.org/mailman/listinfo/sr-users
 
 The IRC channel #kamailio on freenode.net is a place where to meet and discuss
-with other members of Kamailio community.
+with other members of Kamailio community. Additionally there is also a Matrix
+Kamailio community channel available on https://riot.kamailio.dev/.
 
 VI. Contributions
 =================

README.md

@@ -105,6 +105,13 @@ An open IRC discussion channel is managed by the community:
   * irc server: irc.freenode.net
   * irc channel: #kamailio
 
+### Matrix Channel
+
+An open Matrix discussion channel is managed by the community:
+
+  * server: https://riot.kamailio.dev/
+  * Room: https://riot.kamailio.dev/#/room/#kamailio:matrix.kamailio.dev
+
 ## Useful Resources
 
 News: