Maintainers.txt: Add QemuOpenBoardPkg maintainers

Enable maintainers for Platform/Qemu/QemuOpenBoardPkg MinPlatformPkg
based board port.
https://edk2-docs.gitbook.io/edk-ii-minimum-platform-specification/



Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Theo Jehl <theojehl76@gmail.com>
Signed-off-by: Isaac Oram <isaac.w.oram@intel.com>
Acked-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Theo Jehl <theojehl76@gmail.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>