Disambiguate license

[jacobo-doist]

BSD is not enough to specify the license under which the software is released. This change specifies the license to be the 3-clause BSD license.

It also adds a LICENSE file, which was referred to but not included in the repo.

From Appendix: Mapping License Classifiers to SPDX Identifiers | peps.python.org:

Some legacy license classifiers intend to specify a particular license, but do not specify the particular version or variant, leading to a critical ambiguity as to their terms, compatibility and acceptability. Tools MUST NOT attempt to automatically infer a License-Expression when one of these classifiers is used without affirmative user action:

License :: OSI Approved :: Academic Free License (AFL)
License :: OSI Approved :: Apache Software License
License :: OSI Approved :: Apple Public Source License
License :: OSI Approved :: Artistic License
License :: OSI Approved :: BSD License
License :: OSI Approved :: GNU Affero General Public License v3
License :: OSI Approved :: GNU Free Documentation License (FDL)
License :: OSI Approved :: GNU General Public License (GPL)
License :: OSI Approved :: GNU General Public License v2 (GPLv2)
License :: OSI Approved :: GNU General Public License v3 (GPLv3)
License :: OSI Approved :: GNU Lesser General Public License v2 (LGPLv2)
License :: OSI Approved :: GNU Lesser General Public License v2 or later (LGPLv2+)
License :: OSI Approved :: GNU Lesser General Public License v3 (LGPLv3)
License :: OSI Approved :: GNU Library or Lesser General Public License (LGPL)

A comprehensive mapping of these classifiers to their possible specific identifiers was assembled by Dustin Ingram, which tools MAY use as a reference for the identifier selection options to offer users when prompting the user to explicitly select the license identifier they intended for their project.

And then from those mappings:

BSD

- License :: OSI Approved :: BSD License
+ License :: OSI Approved :: BSD 2-Clause Plus Patent License (BSD-2-Clause-Patent)
+ License :: OSI Approved :: BSD 2-Clause "Simplified" License (BSD-2-Clause)
+ License :: OSI Approved :: BSD 3-Clause "New" or "Revised" License (BSD-3-Clause)

[jacobo-doist]

@amix, please take a look at this. I've used the 3-clause BSD license here but, since the base problem is ambiguity, I'd still like to run this change by you.

The other contenders where:

• BSD 2-Clause "Simplified" License
• BSD-2-Clause Plus Patent License

Although there are more under: https://spdx.org/licenses/ I assumed, as per the source above, that "just BSD" would refer to one of these three.

[amix]

@jacobo-doist I'm not an expert here, but this change looks good to me. Feel free to merge or modify as needed.