BaseTools: 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.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://edk2-buildtools.svn.sourceforge.net/svnroot/edk2-buildtools/trunk/BaseTools@2572 c2973edb-eda0-4c78-bc6a-9341b269661f