add shared-random-protocol to PKI specification document!

[david415]

We've got a single field in our consensus document that needs a shared random value... that is... the topology layout of the network. This field controls which mix resides in which topology layer. @mixmasala, @Yawning and I have discussed on our irc channel that we really do need this shared random protocol... however this has NOT yet been added to our PKI specification document:

https://github.com/katzenpost/docs/blob/master/specs/pki.rst