mctp: Add MCTP overview document

This change adds a brief document about the sockets API provided for
sending and receiving MCTP messages from userspace.

This is roughly based on the OpenBMC design document, at:

  https://github.com/openbmc/docs/blob/master/designs/mctp/mctp-kernel.md



Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>