Clarify the mtu option is supported by overlay driver

Signed-off-by: Alessandro Boch <[email protected]>

Author: aboch

engine/userguide/networking/work-with-networks.md

@@ -125,6 +125,8 @@ equivalent docker daemon flags used for docker0 bridge:
 | `com.docker.network.bridge.host_binding_ipv4`    | `--ip`      | Default IP when binding container ports               |
 | `com.docker.network.driver.mtu`                  | `--mtu`     | Set the containers network MTU                        |
 
+The `com.docker.network.driver.mtu` option is also supported by the `overlay` driver.
+
 The following arguments can be passed to `docker network create` for any network driver.
 
 | Argument     | Equivalent | Description                              |