Add LICENSE

[korigod]

As stated at the project website and the package page, holopy is licensed under GPLv3. However, no LICENSE file was created, and it makes license choice unclear. I propose to add the license file because it's automatically recognized by github and then the license is shown in the usual place (near the contributors count). The COPYING file can be removed or not.

[barkls]

Hi korigod,
Thanks for the PR. As you noticed, the file COPYING contains part of the GPLv3 license. I'm not sure why the decision was made to include this rather than a full license file. Any insight @vnmanoharan or @tdimiduk?

[vnmanoharan]

The COPYING file does contain the GPL v3, but for some reason the terms and conditions got truncated, and we didn't notice it. The truncation was not intentional (and is not binding, since the GPL v3 license itself cannot be modified). I would proposed doing as @korigod suggests and replacing the COPYING file with the LICENSE file. We'll need to augment this PR to delete the COPYING file. Thanks, @korigod, for catching this and filing the PR.

[korigod]

@barkls, @vnmanoharan, thanks for the fast replies!

We'll need to augment this PR to delete the COPYING file.

Done.