fix license, contrib docs

CONTRIBUTING.md

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+## Welcome!
+
+We're so glad you're thinking about contributing to an 18F open source project! If you're unsure about anything, just ask -- or submit the issue or pull request anyway. The worst that can happen is you'll be politely asked to change something. We love all friendly contributions.
+
+We want to ensure a welcoming environment for all of our projects. Our staff follow the [18F Code of Conduct](https://github.com/18F/code-of-conduct/blob/master/code-of-conduct.md) and all contributors should do the same.
+
+We encourage you to read this project's CONTRIBUTING policy (you are here), its [LICENSE](LICENSE.md), and its [README](README.md).
+
+If you have any questions or want to read more, check out the [18F Open Source Policy GitHub repository](https://github.com/18f/open-source-policy), or just [shoot us an email](mailto:[email protected]).
+
+## Public domain
+
+This 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](https://creativecommons.org/publicdomain/zero/1.0/).
+
+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.

LICENSE.md

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+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 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 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.