Add a shotkey diode to output of the CV conditioning stages to dump
negative voltage to ground. The maximum voltage range sent to the VC
pins of the AS3360 is 0-2.1V.
Add documentation to the schematic detailing how each chip is configured
and suggesting alternative resistor values for the first CV conditioning
stage for 0-5V and 0-8V operation.
Design uses the [AS3360D](http://www.alfarzpp.lv/eng/sc/AS3360.pdf)
chip. CV inputs are passed through a two inverting opamp stages which
first clip then scale the CV voltage to the operating range of 0-2V.
This results in 0-8V control voltage controlling the full amplitude of
the signal from the input jacks, the outputs are then buffered using a
unity gain opamp to drive the load.
