add properties to README.md
preliminary docs for the log rtype
preliminary docs for the logchunk rtype
preliminary docs for the step rtype
WIP preliminary docs, impl for the slave rtype

** confusion over the general idea of a slave, vs. a slave on a
   particular master.  Is masterid part of the message?  Part of the
   slave object?  Or do we add a way to enumerate the slaves on a master
   and the masters for a slave?