Add full license info for voxel textures

[Montandalar]

Add full license info for voxel textures

Hello,

The textures in the Voxel Demo were included from Minetest Game ("MTG") for Minetest without "full" attribution of the authors. The attribution information needs to be present since the files are under CC BY-SA 3.0.

Although a general attribution was given to the "Minetest Contributors", I thought it was worth noting the original authors of each file that was used, and noting that some are actually under CC0. This info was supplied in a README.

Although CC0 is compatible with CC BY-SA 3.0 and the whole work can be received under CC BY-SA 3.0, it would be good for people to know there are parts they can reuse with less restriction. I think the current wording and linking to "MTG" makes it clear enough if someone wants to attempt such reuse.

[aaronfranke]

I don't think it's legally necessary to list the CC0 textures, but maybe it's a good idea to do it anyway. I'll wait a bit in case other contributors want to give feedback on this PR.

On a technical level, I suggest using ` characters around file names instead of using \_.

[Calinou]

Thanks!