LICENSE: add an explicit ISC license.

The University of Bayreuth's homepage for symmetrica,

  http://www.algorithm.uni-bayreuth.de/en/research/SYMMETRICA/

states that "Symmetrica is public domain." However, that phrase links
to a "Copyright Statement" page that almost verbatim reproduces the
text of the ISC license:

  https://opensource.org/licenses/ISC

To save time in the future, I've included the text of that license
in a new LICENSE file in the repository.