reftable: add LICENSE

reftable: add LICENSE

The objective of this code is to be usable as a C library, so it can be reused
in libgit2.

This is currently using a BSD license as it is the liberal license I could find,
but this could be changed to whatever fits the stated goal above.

This code is currently imported from github.com/hanwen/reftable. Once this code
lands in git.git, the C code will be removed from github.com/hanwen/reftable,
and the git.git code will be the source of truth.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>