facilities.md: Add minimal section on the GSM/cellular networks

There have been some question about the supported bands and availability of SIM cards in the matrix chat, so let's add at least that kind of basic information.

docs/facilities.md

@@ -100,6 +100,27 @@ Heaven is the “partner exchange” of the Chaos events. We try to distribute l
 ## POC
 ## GSM
 
+The c3gsm team has obtained spectrum licenses in several bands.  Right now the plan is to deploy
+* GSM in the 1800 MHz band
+* LTE in the 2600 MHz band
+
+At a lower priority, there might also be UMTS and/or 5G.
+
+### How to get a SIM card
+
+In order to use the private on-site cellular networks, you will need a CCC Event SIM card.
+
+You can either
+* re-use any SIM card from any previous CCC event (congress, camp), or
+* obtain a new SIM card at the event.  We've ordered a large batch of cards.  However, unfortunately there has been some shipping delay.  While the cards have arrived in Germany for customs processing on August 10, they are not expected to arrive at the camp before the morning of August 15 (Day 1).
+
+### What about eSIM?
+
+There was some last minute work ahead of the Camp trying to make eSIM profiles available. It is currently not
+yet known if this will succeed.
+
+By the way: There is going to be a [Talk on eSIM technology](https://pretalx.c3voc.de/camp2023/talk/APYGMR/) in case you want to hear more about the complexities of the eSIM universe
+
 
 ## ISDN (and POTS)