Add better visible License

[toby63]

While I see that there is a license in https://github.com/lxc/linuxcontainers.org/blob/master/CONTRIBUTING.md
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International

It is not so clearly visible at first sight, so maybe you can add the License in a LICENSE-file.

I would have created a pull request for it, but it is unknown to me, whether I can just copy that license or if there is a license for the license 🤔.