Add AlderlakeOpenBoardPkg in the Readme

Reviewed-by: Sai Chaganty <rangasai.v.chaganty@intel.com>
Reviewed-by: Rosen Chuang <rosen.chuang@intel.com>
Acked-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Rosen Chuang <rosen.chuang@intel.com>
Signed-off-by: Saloni Kasbekar <saloni.kasbekar@intel.com>