Added voxel-life demo link and thumbnail

hey, I have a working demo of my voxel-life game, so I went ahead and added it... hope you like it!

Besides the game of life stuff, the demo also has crude multi-voxel copy/paste functionality. I want to figure out a good format for exchanging voxel data, for example one that could also be used with voxel-painter. I guess I need to research to find out what has already been done in that area...