README: add section on network communication

README.md

@@ -38,6 +38,13 @@ Most important steps:
     file
 -   Configure authentication, e.g. [LDAP][4]
 
+## Network communication
+
+This system is designed so that castellum can remain in a secure
+internal network and only the scheduler is accessible from the internet.
+Communication between scheduler and castellum must be possible in both
+directions.
+
 # API
 
 All API requests must send an `Authorization` header with the secret