Added schema zero

Schema Zero is the initial schema that you always go to first.
This schema is immutable, meaning that it will never change in future
revisions.
The idea of Schema 0 is to establish a connection, given a user's
version of the schema in total. This version is the revision of this
repository. The server can then, immediately, know if they are
compatible with the other user of the protocol, and reject the other
user if they are not.

The client is also notified of the server's version if the client is
rejected. This is so that certain programs can choose to use a lower
version of the schema and retry the server when they know the server is
on a lower version than they are.