modified sysadmins role to add internal xmpp service

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

@@ -236,6 +236,16 @@ Below, importance level is evaluated based on:
   - [[How to add checks to our monitoring setup|roles/sysadmins/adding_icinga2_checks]]
 * importance: critical (needed to ensure that other, critical services are working)
 
+## Internal xmpp service
+
+* purpose: an internal xmpp service that can be used by Tails developers and some contributors.
+* access: at the moment everyone that is on the tails-summit mailinglist has and/or can
+  request an account.
+* tools: prosody
+* configuration: 
+  - `tails::prosody` in [[!tails_gitweb_repo puppet-tails]]
+* importance: low 
+
 ## Jenkins
 
 * purpose: continuous integration, e.g. build Tails ISO images from