Change to a CC0-based license. (DOI-USGS#3831)

* Change to a CC0-based license.

* I always forget the link syntax.

LICENSE.md

@@ -0,0 +1,44 @@
+Unless otherwise noted, this project is in the public domain in the
+United States.
+
+It contains materials that originally came from the United States
+Geological Survey, an agency of the United States Department of
+Interior.  For more information on their copyright policies, see
+https://www.usgs.gov/information-policies-and-instructions/copyrights-and-credits
+
+It also contains materials from contributors that have waived their
+copyright interest to the public domain.
+
+Additionally, the authors waive copyright and related rights in the
+work worldwide through the CC0 1.0 Universal public domain dedication.
+
+CC0 1.0 Universal Summary
+-------------------------
+
+This is a human-readable summary of the [Legal Code (read the full
+text)](https://creativecommons.org/publicdomain/zero/1.0/legalcode).
+
+
+### No Copyright
+
+The authors have associated their contributions to the ISIS project
+with this deed, and have dedicated the work to the public domain
+by waiving all of their rights to the work worldwide under copyright
+law, including all related and neighboring rights, to the extent
+allowed by law.
+
+You can copy, modify, distribute and perform the work, even for
+commercial purposes, all without asking permission.
+
+
+### Other Information
+
+In no way are the patent or trademark rights of any person affected
+by CC0, nor are the rights that other persons may have in the work
+or in how the work is used, such as publicity or privacy rights.
+
+Unless expressly stated otherwise, the authors who have associated
+the ISIS project with this deed make no warranties about the work,
+and disclaim liability for all uses of the work, to the fullest
+extent permitted by applicable law. When using or citing the work,
+you should not imply endorsement by the authors.