SAFETY: Document correct systemd snippet path
SAFETY: Document correct systemd snippet path

If you're using systemd with something other than systemd-networkd to
bring up your network (e.g. a service named some.service), the systemd
configuration snippet should go into /etc/systemd/system/some.service.d/
and *not* into /etc/systemd/some.service.d/ as the README.md previously
instructed.

In the event that corridor-init-forwarding.service failed for some
reason, such as a broken iptables binary, the misplaced snippet would
be ignored and the network would be brought up unfiltered. (corridor
would then also not enable IPv4 forwarding, but maybe your system has
been set up to enable it independently?)

rustybird/corridor,adrelanos/corridor