ManageabilityPkg: Add Readme file

Add Readme file of edk2 platform ManageabilityPkg.

Signed-off-by: Abner Chang <abner.chang@amd.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Nickle Wang <nicklew@nvidia.com>
Cc: Igor Kulchytskyy <igork@ami.com>
Cc: Abdul Lateef Attar <abdattar@amd.com>
Reviewed-by: Abdul Lateef Attar <AbdulLateef.Attar@amd.com>
Reviewed-by: Nickle Wang <nicklew@nvidia.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>
Reviewed-by: Abdul Lateef Attar <abdattar@amd.com>