Add LICENSE file

In order for individuals to use or modify this project legally, it must have a license.

Relevant links:
* [Choose an open source license](https://choosealicense.com/)
* [GitHub Help: Licensing a repository](https://help.github.com/articles/licensing-a-repository/)
* [GitHub Help: Adding a license to a repository](https://help.github.com/articles/adding-a-license-to-a-repository/)