Better explain Draco extension used/required

[pjcozzi]

From @bghgary in KhronosGroup/glTF-Sample-Models#145 (comment)

I think we can add a couple of minor things to avoid confusion.

• Add an short explanation to the extension required part in the spec and indicate that the only difference is that bufferView is missing from the accessor.

• Add a sample model of Draco with a fallback where Draco is only in extensionsUsed. We can also add this to the asset generator project instead if that's better.

CC @FrankGalligan