document the matrix bridge

howto/irc.md

@@ -85,6 +85,33 @@ for example with:
 
     access @tor-ircmasters del OLDADMIN
 
+## Using the Matrix bridge
+
+Since mid-April 2021, many `#tor-*` channels are bridged (or
+"[plumbed](https://matrix.org/bridges/)") between the [OFTC IRC network](https://oftc.net) and the
+[Matrix.org](https://matrix.org/) home server. This is still somewhat of an experimental
+status (e.g. `#tor` itself is not bridge yet), but so far it seems to
+be working well.
+
+Internal channels are not bridge directly yet either, but they can
+still be joined if you authenticate with `NickServ` through the IRC
+bridge bot. The following might get you going:
+
+    !nick <yournick>
+    !storepass <yourpass>
+    !join #secret-channel <channel password>
+
+The [IRC bot documentation](https://matrix-org.github.io/matrix-appservice-irc/latest/) might provide more information on that
+functionality.
+
+Also note that you can join *any* channel on bridged IRC networks
+(Freenode and OFTC) through the "Portal rooms" functionality. For
+example, `#freenode_#channelname:matrix.org` corresponds to
+`#channelname` on Freenode.
+
+See also the [bridges documentation](https://matrix.org/bridges/) for more information on the
+different types of bridges on Matrix.
+
 ## Pager playbook
 
 <!-- TODO: information about common errors from the monitoring system and -->