Fix networking advice.

The blog post [Introduction to networkd, network managment from systemd][post]
contradicts the [Network Configuration][docs] documentation.

The blog post is right. It wasn't until I swapped the numbers around and ran
with `10-static.network` and `20-dhcp.network` that my interfaces switched
from DHCP to static addressing.

I've also added advice on how to check status for those new to `systemd`.

[post]: http://coreos.com/blog/intro-to-systemd-networkd/
[docs]: https://coreos.com/docs/cluster-management/setup/network-config-with-networkd/