README: Add section addressing motherboard-support FAQ

This is a common enough question on Discord that we should probably make
it easier for people to find the answer themselves (or at least have an
easy thing to refer people to when it inevitably arises anyway).

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Change-Id: I54757f0902bfedca86d41cab0fc55b39cb7dfc1e