planning: [Meeting Planning Pages] info on IRC backchannel

Author: dbaron

planning.txt

@@ -2,6 +2,8 @@
 
 This is the CSS Working Group's planning space. See also [[planning:hosting]].
 
+All meetings use IRC for live minuting and as a backchannel: the #css channel on [[https://www.w3.org/Project/IRC/|the W3C IRC server]].
+
 ===== Weekly Telecons =====
 
 [[https://lists.w3.org/Archives/Member/w3c-css-wg/2017OctDec/0076.html|teleconference details and times for weekly teleconferences]]