add swarm instructions

docker-compose.yml

@@ -1,5 +1,9 @@
 version: '3'
 
+# Environment variables are replaced with definitions in .env, when run with:
+#
+#  env $(cat .env | grep ^[A-Z] | xargs) docker stack deploy --compose-file docker-compose.yml server
+
 networks:
   default:
     driver: overlay