We are currently focusing on the 4.2 editor and runtimes. We'll probably not update the JSON format documentation until later. We'll probably write Spine User Guide documentation first. This is because there are much more people are using the editor. Not very many people care about the JSON format directly because they use our runtimes which handle that.
Please note that those people who use their own runtimes are going to have to implement their own physics. If your runtime uses our physics code from our Spine Runtimes, then our runtimes license applies to your code and to any products made with your code. Our license says that anyone using our Spine Runtimes code in their products needs a Spine license. If you don't want our licensing applied to your code and products, then you need to write your own physics without looking at our code at all. This is called a "clean room" implementation. This applies to other code in our runtimes -- special care must be taken not to copy any of our code, else our licensing applies.
If you do put our physics code into your runtime code, then our license applies. In that case, you would save a whole lot of effort by simply using our runtimes. Then you don't need to care about the JSON format at all. We put a LOT of effort into the runtimes to save you all the trouble of having to implement your own. Our licensing should not be a problem, it just ensures that only people who license Spine can benefit from our runtimes.
