README: point to Matrix

Users are seeing the information to Freenode, which is
probably the least correct option now. RFC94 isn't accepted
yet, and we can update this README as soon as immediately
if something changes.

README.md

@@ -21,10 +21,10 @@
 # Community
 
 * [Discourse Forum](https://discourse.nixos.org/)
-* [IRC - #nixos on freenode.net](irc://irc.freenode.net/#nixos)
+* [Matrix Chat](https://matrix.to/#/#community:nixos.org)
 * [NixOS Weekly](https://weekly.nixos.org/)
 * [Community-maintained wiki](https://nixos.wiki/)
-* [Community-maintained list of ways to get in touch](https://nixos.wiki/wiki/Get_In_Touch#Chat) (Discord, Matrix, Telegram, other IRC channels, etc.)
+* [Community-maintained list of ways to get in touch](https://nixos.wiki/wiki/Get_In_Touch#Chat) (Discord, Telegram, other IRC channels, etc.)
 
 # Other Project Repositories