Readme.md: Add Intel platforms

Adds information to Readme.md for Intel platforms recently
moved to the edk2-platforms repository.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>

Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>