Extract the systemd docs from various places and add a little more

@lsm5 @philips @jfrazelle I don't grok why the fedora `docker.services` file is different from the one in https://github.com/docker/docker/blob/master/contrib/init/systemd

if you're ok with it, I'll update the one in our repo - or maybe we should delete them? 
I think this needs some context. Why am I doing this? Are there multiple steps to this process and this is the first one? I think we need a brief overview/summary that explains to the reader what's coming.