edk2-platforms: Remove Contributions.txt and update Readme.md

* Remove the Contributions.txt files from the edk2-platforms repository
  that contained the TianoCore Contribution Agreement.

* Update Readme.md in the root of the edk2-platforms repository to state
  that BSD+Patent is the preferred license for the EDK II project.

* Point to the Readme.md file in the edk2 repository for details on how
  to contribute along with the commit message format.

* Point to the Readme.md file in the edk2 repository for the meaning of
  Signed-off-by that means the contributor certifies compliance to the
  Developer's Certificate of Origin 1.1.
  https://developercertificate.org

https://bugzilla.tianocore.org/show_bug.cgi?id=1373

This change is based on the following emails:

  https://lists.01.org/pipermail/edk2-devel/2019-February/036260.html
  https://lists.01.org/pipermail/edk2-devel/2018-October/030385.html

RFCs with detailed process for the license change:

  V3: https://lists.01.org/pipermail/edk2-devel/2019-March/038116.html
  V2: https://lists.01.org/pipermail/edk2-devel/2019-March/037669.html
  V1: https://lists.01.org/pipermail/edk2-devel/2019-March/037500.html



Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Thad Gillispie <thad.gillispie@intel.com>
Cc: Daocheng Bu <daocheng.bu@intel.com>
Cc: Isaac W Oram <isaac.w.oram@intel.com>
Cc: Maciej Piwko <maciej.piwko@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Michael Kubacki <michael.a.kubacki@intel.com>
Cc: Shifei A Lu <shifei.a.lu@intel.com>
Cc: Xiaohu Zhou <bowen.zhou@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Sai Chaganty <rangasai.v.chaganty@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>