Add a License

[Ryuno-Ki]

Hey,

thanks for your cool work!
Sadly we wouldn't be able to reuse it for presentations about state charts, because this repo has no license attached.

Thus all rights are reserved.

Would you be open to attach a license?

[mogsie]

Thanks for this issue. I am thinking it over, since I might turn some of the content here into a book, I want to tread a bit carefully. Hang in there :)