Clarify License Agreement

Hi! I'm interesting in using this library in a project I'm working on, but am hesitant to do so because of a lack of a license agreement. In the cargo.toml https://github.com/askanium/rustplotlib/blob/master/Cargo.toml, it specifies the license as "MIT OR Apache 2.0". It would be great to clarify what this means and add a LICENSE.md to the root of the project. 