Maintainers.txt: Add StandaloneMmPkg and maintainers

Maintainers.txt: Add StandaloneMmPkg and maintainers

This patch adds maintainers, reviewer and directory for the
StandaloneMmPkg. This package will host an implementation of Standalone
Management Mode as specified in the Platform Initialization (PI)
Specification, Volume 4: Management Mode Core Interface.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Reviewed-by: Jiewen.yao@intel.com
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>