Specify Service Registry contract

[pirapira]

In service_contracts.rst, most service contracts are described in detail (at least several paragraphs). However, Service Registry contract is only described in one short paragraph. This issue keeps track of specifying the Service Registry contract(s) in service_contracts.rst to the extent that anybody would implement something similar following the spec.

[pirapira]

The decision on the auction format here is waiting until trustline chooses an auction format. I hear trustline's decision happens by this weekend.

[pirapira]

@stefante now says we are waiting for the implementation of auctions. I've sent an email to Andreas so he is aware that we are waiting.