policy for acceptable licenses from contributors

We should have a policy statement about what licenses we will accept for contributions to Core Tor. Maybe it should go into CodingStandards.md or similar.

It seems like we currently want BSD-licensed code (3-clause?) and maybe don't want copylefted stuff?

Maybe also clarify that patches to an existing file are assumed to be copyright-assigned to us or licensed under the same license as the original file.

Bonus points if there's a well-written rationale for our licensing choices. (But maybe that should be a separate ticket.)