Add CC0 license

LICENSE.md

+As a work of the United States Government, this project is in the
+public domain within the United States.
+
+Additionally, we 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 person who associated a work with this deed has dedicated the work to the
+public domain by waiving all of his or her 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. See **Other Information** below.
+
+## 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 person who associated a work with this
+  deed makes no warranties about the work, and disclaims 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 author
+  or the affirmer.