add section about on-topic channels

* from matrix space view #libregaming-communities:freedombox.emorrp1.name
* omits #libreplanet-gaming as inactive and moves affiliates to top

chat-with-us.md

@@ -50,6 +50,37 @@ The topic for our main channel is:
 
 To join one such room called `ROOM`, configure your client according to the instructions below.
 
+## Communities
+
+For on-topic chat about Libre Gaming, these are the active communities we are aware of.
+
+- [FreeGamedev.net](https://freegamedev.net): bring players and developers together like the old #freegamer on Freenode
+  [matrix](https://matrix.to/#/#xmpp_#general_irc.freegamedev.net:matrix.org)
+  [xmpp](xmpp:#general@irc.freegamedev.net?join)
+  [irc](ircs://irc.freegamedev.net/#general)
+  [archive](https://view.matrix.org/room/!YdOflpMHoyrSVDkJih:matrix.org/)
+- [Libre Game Night](https://lgn.xwx.moe/): play and discuss libre games such as Hedgewars, Mindustry, Freedoom
+  [matrix](https://matrix.to/#/#libregamenight:libera.chat)
+  [xmpp](xmpp:#libregamenight#libera.chat@matrix.org?join)
+  [irc](ircs://irc.libera.chat/#libregamenight)
+  [archive](https://view.matrix.org/room/!rgtWjnTtagQNSTFLWy:libera.chat/)
+- [onFOSS-LAN](https://onfoss.hribhrib.at/): online LAN-Party event day
+  [matrix](https://matrix.to/#/#xmpp_onfoss_conference.hribhrib.at:matrix.org)
+  [xmpp](xmpp:onfoss@conference.hribhrib.at?join)
+  [archive](https://view.matrix.org/room/!wZHmKuwbDAgPrepPeC:matrix.org/)
+- [opensource_gaming](https://twitch.tv/opensource_gaming): twitch channel to bring attention to a lot of great Open Source projects
+  [matrix](https://matrix.to/#/#generalosg:matrix.org)
+  [xmpp](xmpp:#generalosg#matrix.org@matrix.org?join)
+  [archive](https://view.matrix.org/room/!vqlhRElKHgxMUOxRuJ:matrix.org/)
+- *SMAC*: чат без смысла и цели
+  [matrix](https://matrix.to/#/#xmpp_smac_conference.bitcheese.net:matrix.org)
+  [xmpp](xmpp:smac@conference.bitcheese.net?join)
+  [archive](https://view.matrix.org/room/!DIoqkjRprogKLnbFkO:matrix.org/)
+- *Free, Libre, and Open Source Gaming*
+  [matrix](https://matrix.to/#/#libregaming:matrix.org)
+  [xmpp](xmpp:#libregaming#matrix.org@matrix.org?join)
+  [archive](https://view.matrix.org/room/!xbJbNDiHWlkrtqJbPo:matrix.org/)
+
 ## Matrix
 
 Room name: `#libregaming-ROOM:freedombox.emorrp1.name`