Information about loading strategies for glTF assets

[javagl]

In response to KhronosGroup/glTF#755 (comment) , I created a gist summarizing the thoughts and discussions about (asynchronous) loading strategies for glTF: https://gist.github.com/javagl/bfde5cfab4240843120ed6eb38f4af87

This might become a section in the main tutorial. But since the tutorial until now is rather on a conceptual level, and not so much concerned with the implementation (say, of a viewer/renderer), this might also become a section of a (new?) tutorial that focuses on the implementation.