MagicD ANET-A8 build plate support

the MagicD ANET-A8 (from Amazon) is a fairly decent variation of the ANET-A8 configuration that uses 2040 extrusion for its' structure ... decent, except for the yaw support of the build plate around the Y-axis ... they use a wheeled trolley design to translate the Y-axis, but they build the trolley across the 20mm width of the 2040 extrusion, which results in the build plate being very unstable (wobble-ly) in rotation about the Y-axis ... this fix adds support rollers to the underside of the bu