Document cjdroute.conf

[ansuz]

Define all attributes used within the file, link to a JSON spec, and specify where cjdroute.conf differs from conventional JSON.

[ansuz]

I think this commit takes care of it, but another pair of eyes would be great.

[cschmittiey]

Something might need to be added to reflect that there are two UDPInterfaces now, one for ipv6 and one for ipv4.

[ansuz]

Information regarding cjdroute.conf is scattered:

1. https://github.com/hyperboria/docs/blob/master/notes/cjdroute-conf.md
   • I'm considering moving this to cjdns/cjdroute-conf.md, but this might break links.
   • this goes back to the problem raised in this issue, how can we structure our documentation so as to make links less fragile?
2. https://github.com/hyperboria/docs/blob/master/cjdns/peering-over-UDP-IP.md
   • this should probably be a section in #1, but before moving it, I'd like to address the dead link issue