Add tianocore required text files

Add License.txt, copied verbatim from commit efd798c1eb of
  https://git.linaro.org/uefi/OpenPlatformPkg.git
Add Contributions.txt from verbatim from commit 7d3921f893 of
  https://github.com/tianocore-docs/edk2-TemplateSpecification


Add Maintainers.txt
  Adapted from edk2-non-osi master branch.
Add Readme.md

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>