Update README

* [x] Added more information about Vispy's current status.
* [x] Added links to developers' home pages.
That perspective actually suggests a different top-level structure:
```
vispy.visuals   (all visual subclasses here)
vispy.visuals.transforms
vispy.visuals.shaders
vispy.scene  (contains scenecanvas, events, cameras, systems...)
```
But that is neither on-topic nor pressing  :)