Adds an open source license (MIT)

I couldn't find a reference to an open source license in the project.

But after talking with the staff at Minas Gerais I realized that they want to see this project re-used, and that they are fine with an attribution-only license.

Could we verify this and add a license?

Some references:
* [What happens if I don't choose a license?](https://help.github.com/articles/open-source-licensing#what-happens-if-i-dont-choose-a-license)
* https://tldrlegal.com/license/mit-license
* http://choosealicense.com