Add LICENSE file

[vazhnov]

Hello!

Thank you for a great project, published under opensource license!

Could you please add a file LICENSE or COPYING with a license text, so the forks (like https://github.com/yeokm1/graphics-gremlin-hdmi) wouldn't loose it by a mistake?

It is easy to add the file with GitHub, it has the license template.

If you want, I can help you.