Authority-discovery mechanism isn't documented

[tomaka]

All the validators of a chain have to publish on the Kademlia DHT a signed record containing their PeerId and addresses at which they are reachable at a key corresponding to their validator key.

The precise format is somewhere here: https://github.com/paritytech/substrate/tree/master/client/authority-discovery

Note that IMO we shouldn't put the name "authority discovery" in the spec, instead we should just mention that validators have to publish a record in the DHT without giving a name to that mechanism.

[heversonbr]

Topic added by PR669.