Explain briefly why builtin services are not containerized

README.md

@@ -50,6 +50,11 @@ and in README files for individual Ansible roles:
 * [Log management and analysis](roles/log-collector/README.md)
 * [Authoritative public DNS](roles/dns/README.md)
 
+Built-in services are currently implemented with Ansible roles, and do
+not run in containers. But this is just an implementation detail, and
+in the future they could be moved to containers without requiring any
+changes in the clients.
+
 # Requirements
 
 On the local machine (the one that will run Ansible), you're going to