Add initial license to README

Author: dshean

README.md

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 # ICESAT2_hackweek_tutorials
 Combined repository for final tutorial material from 2019 ICESat-2 HackWeek at the Univeristy of Washington
+
+## License
+The content of this project itself is licensed under the [Creative Commons Attribution 3.0 Unported license](https://creativecommons.org/licenses/by/3.0/), and the underlying source code used to format and display that content is licensed under the [MIT license](LICENSE.md).