Add mplatform/mquery multi-platform enablement info
Add mplatform/mquery multi-platform enablement info

Add note in README.md about mplatform/mquery itself being a manifest
list image.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@gmail.com>

estesp/mquery,estesp/mquery