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.

LICENSE

+Copyright (c) 2010, symmetrica@symmetrica.de
+Copyright (c) 2020, The Sage Development Team
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.