edk2: Remove Contributions.txt and update Readme.md

edk2: Remove Contributions.txt and update Readme.md

* Remove the Contributions.txt file from the root of the edk2 repository
  that contained the TianoCore Contribution Agreement.

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

* Move the portions of Contributions.txt to Readme.md that describe
  how to contribute along with the commit message format.

* Add to Readme.md that Signed-off-by means that 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

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>