Improve doc of the diff method

[paulocheque]

The diff method returns a dictionary of patches, nodes and arrays. It would be great to have a simple explanation about the output object of that method.

This one just present some numbers and it is not clear: https://github.com/Matt-Esch/virtual-dom/blob/master/vtree/README.md

ps: I am trying to get an array with the keys of all nodes with changes