Document the need to EXPOSE the proxied port

README.md

@@ -14,6 +14,8 @@ 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/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`.
+
 Provided your DNS is setup to forward foo.bar.com to the a host running nginx-proxy, the request will be routed to a container with the VIRTUAL_HOST env var set.
 
 If you are using `boot2docker` start `nginx-proxy` with: