Specify script's licence

[mjenczmyk]

Should we specify which licence does the script use? We could use some open source licence.

I would go for GPL 3.0 licence (as it's free), but a more permissive and non-infectious licence should also be fine (i.e. MIT licence).

[Lukasz Uszko]

Added at: f9761f0

[mjenczmyk]

Thanks!