Update README

* [x] Added more information about Vispy's current status.
* [x] Added links to developers' home pages.
The scenegraph is little more than a way of automating rendering and interaction with multiple visuals..

In my opinion, the core of vispy will be its library of visuals. Most people who use vispy will be looking foremost for high-level visuals like surface plots, volume rendering, etc. Most users probably will not care (at first, anyway) that we are using opengl / gloo, support multiple backends, or have a scenegraph architecture. All they will want to know is "can it draw X easily", and for that we need a large library of prominently-featured visuals.