Document relation to network_mode: service:container

docs/linked-containers.md

@@ -3,3 +3,5 @@ Watchtower will detect if there are links between any of the running containers
 For example, imagine you were running a _mysql_ container and a _wordpress_ container which had been linked to the _mysql_ container. If watchtower were to detect that the _mysql_ container required an update, it would first shut down the linked _wordpress_ container followed by the _mysql_ container. When restarting the containers it would handle _mysql_ first and then _wordpress_ to ensure that the link continued to work.
 
 If you want to override existing links, or if you are not using links, you can use special `com.centurylinklabs.watchtower.depends-on` label with dependent container names, separated by a comma.
+
+When you have a depending container that is using `network_mode: service:container` then links or the `depends-on` label is required for the restart of the depending container to work.