Add examples of extending superclass properties and methods

Unlike props/derived/... from ampersand-state, view properties such as `bindings`  and `events`  aren't merged with the ones from the superclass. This is something I occasionally want to do.
Similarly, I sometimes want to extend behaviour of a base class in `render()` and `initialize()`.

Being relatively new to this, it wasn't immediately obvious to me how to do this, and an example would have helped a lot, so I documented the way I ended up doing this.

Some of this may border on basic JavaScript knowledge (although knowing what View.extend() does behind the screens isn't), but I still think it has a place somewhere in some documentation, and this is the best place I could find.