Add a contributing nice

CONTRIBUTING.md

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+## Public domain
+
+The project is in the public domain within the United States, and copyright and related rights in the work worldwide are waived through the [CC0 1.0 Universal public domain dedication][CC0].
+
+All contributions to this project will be released under the CC0 dedication. By submitting a pull request, you are agreeing to comply with this waiver of copyright interest.
+
+[CC0]: http://creativecommons.org/publicdomain/zero/1.0/