Information about loading strategies for glTF assets

In response to https://github.com/KhronosGroup/glTF/issues/755#issuecomment-457017607 , 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.

Read the linked gist and compare its loading-strategy discussion with the existing main tutorial. Determine whether the material belongs in that tutorial or a new implementation-focused tutorial; done means documenting the agreed placement and incorporating the relevant asynchronous loading guidance.

Written by the indexing model from the issue text.