Add "Why can't I rename my homeserver?"

supporting-docs/guides/2015-08-19-faq.md

@@ -571,6 +571,12 @@ Synapse is *very* database dependent (as of Oct 2015; this is improving in the n
 
 Data is only shared between servers of participating users of a room. If all users in a room are on your server, no data is shared with other servers.
 
+##### Why can't I rename my homeserver?
+
+Currently, the homeserver name is assumed never to change. This means that if you rename your server, other servers will think it's a different server. 
+
+Perhaps in the future we will add an API for changing the homeserver name, but for now this is not supported.
+
 ### Clients
 
 ##### Where can I find a mobile app?