Document new meeting reminder service

wiki/src/contribute/working_together/roles/sysadmins.mdwn

@@ -333,6 +333,19 @@ Below, importance level is evaluated based on:
   `tails::spamassassin` classes in [[!tails_gitweb_repo puppet-tails]]
 * importance: high (at least because WhisperBack bug reports go through this MTA)
 
+<a id="meeting-reminder"></a>
+
+## Meeting reminder
+
+* purpose: send email reminders, for example about upcoming meetings
+* access: not applicable
+* configuration:
+  - to add a new reminder, or modify an existing one:
+    - `tails::meeting::reminders` in [[!tails_gitweb_repo puppet-tails]]
+    - templates are in `files/meeting/` in [[!tails_gitweb_repo puppet-tails]]
+  - implementation: `tails::meeting`, `tails::meeting::reminder`
+* importance: to be defined
+
 <a id="mumble"></a>
 
 ## Mumble