Update README.md

Author: jpomykala

README.md

@@ -16,7 +16,7 @@ Then start any containers you want proxied with an env var `VIRTUAL_HOST=subdoma
 
     $ docker run -e VIRTUAL_HOST=foo.bar.com  ...
 
-The containers being proxied must [expose](https://docs.docker.com/engine/reference/run/#expose-incoming-ports) the port to be proxied, either by using the `EXPOSE` directive in their `Dockerfile` or by using the `--expose` flag to `docker run` or `docker create`.
+The containers being proxied must [expose](https://docs.docker.com/engine/reference/run/#expose-incoming-ports) the port to be proxied, either by using the `EXPOSE` directive in their `Dockerfile` or by using the `--expose` flag to `docker run` or `docker create` and be in the same network. By default, if you don't pass the --net flag when your nginx-proxy container is created, it will only be attached to the default bridge network. This means that it will not be able to connect to containers on networks other than bridge.
 
 Provided your DNS is setup to forward foo.bar.com to the host running nginx-proxy, the request will be routed to a container with the VIRTUAL_HOST env var set.