Add USDS boilerplate license and contributing details

CONTRIBUTING.md

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+## Welcome!
+
+We're so glad you're thinking about contributing to a USDS open source
+repository! If you're unsure or afraid of anything, just ask or submit the
+issue or pull request anyways. The worst that can happen is that you'll be
+politely asked to change something. We appreciate any sort of contribution,
+and don't want a wall of rules to get in the way of that.
+
+Before contributing, we encourage you to read our CONTRIBUTING policy (you are
+here), our LICENSE, and our README, all of which should be in this repository.
+
+## 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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+This project includes a mix of the following:
+
+* Open source works that are not in the public domain
+* Open source work by the U.S. government that is in the public domain
+
+## Parts of this project that are not in the public domain
+
+This site is based on the U.S. Web Design System (USWDS), which also
+includes works under the SIL Open Font License, MIT license, and public
+domain. The full license details for those assets are described in the
+[USWDS license file](https://github.com/uswds/uswds/blob/master/LICENSE.md).
+
+## The rest of this project is in the worldwide public domain
+
+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](https://creativecommons.org/publicdomain/zero/1.0/).
+
+### 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.
+
+#### 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.
+
+### Contributions to this project
+
+As stated in [CONTRIBUTING](CONTRIBUTING.md), 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.