update readme
BoxTree:

- use coordinates in Node to avoid calculation in query()
- inline node overlap check in query
refactor: extract MarkerRenderer from ItemizedLayer

- move ItemizedLayer stuff to MarkerLayer
- rename ItemizedIconLayer to ItemizedLayer
rename getViewport/Animator/Layer() to viewport/animator/layers()

- these are composite parts of Map
refactor: Viewport.getViewBox() -> getBBox()