docs: Ported Design Doc to OpenBMC

This is a port of the original Google Design Document for Swampd, now
phosphor-pid-control. This document needs updates to match the latest
change

docs: Ported Design Doc to OpenBMC

This is a port of the original Google Design Document for Swampd, now
phosphor-pid-control. This document needs updates to match the latest
changes pertaining to dbus configuration.

Change-Id: Idcef393d06547b108655d08236b1949c04b7367b
Signed-off-by: Patrick Venture <venture@google.com>

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