Add a LICENSE file (BSD)

[martin-g]

The README says that the repository license is BSD -

MCScanX/README.rst

Line 4 in b1ca533

:License: `BSD <http://creativecommons.org/licenses/BSD/>`_

With this PR I add a LICENSE file for the same.

[tanghaibao]

@martin-g

While BSD LICENSE is no problem with me, I'll need to defer the license choice to the owner of this repo.
If @wyp1125 is okay then we should be able to do this.

Haibao

[tanghaibao]

Actually the program currently lists BSD in its README, so I'll assume that this is fine, but @wyp1125 if you prefer otherwise.