Readme.md: Add a reference to SimicsOpenBoardPkg

Adds a link to SimicsOpenBoardPkg in the list of supported Intel
platforms.

Cc: Wei David Y <david.y.wei@intel.com>
Cc: Agyeman Prince <prince.agyeman@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>