Clarifying i3/i3v heat bed and heat bed relay wiring

[printbus]

Just to clarify, nothing in the writeup encouraged anyone to use a relay on the extruder heater, but that's interesting to know.

EDIT: After pondering this, there really is no technical reason why one couldn't put a relay in the extruder heater circuit. The extruder heater circuit, however, can turn on and off at a pretty high rate - multiple times a second. That would lead to a lot of annoying relay click noise. Perhaps OME's relay froze up or there was some sort of wiring problem.