facilities.md: Add some general DECT/POC information

I'm not a POC member, but just adding general knowledge known from prior
events, which is better than an empty section.

docs/facilities.md

@@ -98,6 +98,18 @@ Events of this size are only possible with your help, so please [participate](ht
 Heaven is the “partner exchange” of the Chaos events. We try to distribute lifeforms who want to help out at our events (”angels”) among the work that is brought to us by other teams. At the same time, we do the best we can to make sure our angels have fun doing what they do and are doing well. We are always available for our angels as contact persons and help them with questions, just as we try to find good and pragmatic solutions for the teams for the work that arises.
 
 ## POC
+
+As it is customary for CCC events for ages, the POC (Phone Operation Centre) powered by eventphone is again
+operating a site-wide DECT network.
+
+In order to use the network, please
+* create an account (or log in using an existing account) on [GURU](https://guru3.eventphone.de/)
+* register your phone(s) in the GURU web interface
+* bring your DECT phone (only EU band phones for the DECT band work; US-DECT operates in 2.4 GHz and is not compatible)
+* once on-site, go to the POC helpdesk and ask a friendly angel to activate it
+
+More information can be found at the resources linked from the [eventphone support page](https://guru3.eventphone.de/support/)
+
 ## GSM