add a license file

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This project should specify a license to allow contributions to and the distribution of this library. GitHub encourages the developers to specify one:

You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work. If you're creating an open source project, we strongly encourage you to include an open source license. The Open Source Guide provides additional guidance on choosing the correct license for your project.

Popular choices include the Apache License 2.0 or the MIT License