COPYING: Relicense under the terms of the ISC license

The current license seems to be an *ad-hoc* one, based on the terms of the JSON license.
If this is indeed the case, please consider this *pull request*, which proposes
relicencing under the terms of the ISC license.

The ISC license seems to be functionally equivalent to the current one,
but is more established, removing opportunities for confusion.

The Markdown-formatted version of the license was taken from
<https://github.com/nevir/readable-licenses/blob/master/markdown/ISC-LICENSE.md>