add trademark policy at /trademark

This policy was written by Software Freedom Conservancy's
legal counsel on behalf of the Git Project. It needs to be
posted publicly; this patch starts by making it available,
and we'll link to it in future commits.
about: link to trademark page from "free and open source"

This seems like the most sensible place to link to it.
Merge pull request #401 from git/peff/trademark-policy

add trademark policy