EDK II Packages: Add Contributions.txt and License.txt files

Contributions.txt documents the contribution process for all
tianocore projects. The conents of Contributions.txt should
match in all cases.

License.txt is a per-project document showing the license terms
used by that project.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13187 6f19259b-4bc3-4df7-8a09-765794883524