Document usage through voxel-engine-stackgl

[deathcap]

The main usage of voxel-plugins is via https://github.com/voxel/voxel-engine-stackgl, but this readme documents standalone usage - should point to voxel-engine-stackgl for the common usage, which hides the details of creating the plugin system (also, should document 'loaders' option preferred over 'require': #8 - and how can be extended after creation with this.loaders)