Add strong recommendation for portal-external tag (azimuth-cloud#181)

Author: MoteHue

docs/configuration/04-target-cloud.md

@@ -110,6 +110,12 @@ to discover the networks it should use, and the tags it looks for are `portal-in
 `portal-external` for the internal and external networks respectively. These tags must be applied
 by the cloud operator.
 
+!!! tip
+
+    It is strongly recommended that you set the `portal-external` tag on an appropriate external network,
+    even if you only have one external network, to avoid issues if new external networks are added to the
+    cloud at a later date.
+
 If it cannot find a tagged internal network, the default behaviour is for Azimuth to create an
 internal network to use (and the corresponding router to attach it to the external network).