required env var not documented on README

Hi,

The following env vars are required but not documented on the README file:

```
# Wait for SecFlood Interfaces to become available and then update config
echo "waiting for interfaces to become available..."
until ip link show dev "$SECFLOOD_INTF_INSIDE" >/dev/null 2>&1; do
	sleep 2
done
until ip link show dev "$SECFLOOD_INTF_OUTSIDE" >/dev/null 2>&1; do
	sleep 2
done
```