EDK II Contributions.txt: Note acceptable contribution licenses

We strongly prefer that contribtions be offered using the same license
as the project/module. But, we should document other acceptable
licenses for contributions.

This will allow package owners to more easily know if they can accept
a contribution under a different source license.

NOTE: This does not modify the wording of the "TianoCore Contribution
      Agreement 1.0" section

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

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/OvmfPkg@15892 6f19259b-4bc3-4df7-8a09-765794883524