doc: Add section about creating new docker network

Flannel doesn't need to replace deamon0 network in Docker and does work even if the you create new docker network that uses flannel.

Documentation/running.md

@@ -98,6 +98,12 @@ docker daemon --bip=${FLANNEL_SUBNET} --mtu=${FLANNEL_MTU} &
 
 Systemd users can use `EnvironmentFile` directive in the `.service` file to pull in `/run/flannel/subnet.env`
 
+If you want to leave default docker0 network as it is and instead create a new network that will be using flannel you do so like this:
+```bash
+source /run/flannel/subnet.env
+docker network create --attachable=true --subnet=${FLANNEL_SUBNET} -o "com.docker.network.driver.mtu"=${FLANNEL_MTU} flannel
+```
+
 ## CoreOS integration
 
 CoreOS ships with flannel integrated into the distribution.