New Sea level Use case

I've added (and self-assigned) another sea-level use case to include,
which happens to fall out from Dave McCalpin's submition to the XRI TC
on Trusted Resolution (identified as XriAuthorityChainRegistration).
Essentially, the motivation is where delegation points exist within the
'authority-part' of the xri construct, key material needs to be
exchanged (and trust processing need occur), between the authorities, as
they are asserting the relationship(s) between the child node and the
authoritative status of the registerd XRI Authority.

Hopefully, when i complete the use case, it will be clearer than
depicted here ;-)

--- peterd