ReadMe.md: Add Minimum Platform details

Adds details on the EDK II Minimum Platform design for Intel
platforms.

* Overview of Minimum Platform
* Board package purpose and conventions
* Stage boot concept and control
* Minimum Platform firmware solution stack overview
* Updates build instructions for all <xxx>OpenBoardPkgs
* Adds information for the ClevoOpenBoardPkg
* Adds planned activities and ideas for the future

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Isaac W Oram <isaac.w.oram@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>