Add spacer plate to BoM for Nema 23 Z Axis

[jaymis]

If you're using a Nema 23 motor on the Z axis, you need a spacer plate underneath the spindle mount, so the spindle clears the Z motor mount.

https://discordapp.com/channels/648972213734604807/648972213734604810/683188554351771674

https://discordapp.com/channels/648972213734604807/648972213734604810/706944254987075694

An elegant solution may be a design that could be printed and then replaced with a machined aluminium part, or otherwise a template/hole drilling pattern so people can fabricate their own from aluminium plate.