Suggest in the documentation to run ```ETS_TOOLKIT="qt4" python examples/demo/demo.py``` ?

Dear all,

Just a tiny suggestion to tell curious users to run the following command:

```
ETS_TOOLKIT="qt4" python examples/demo/demo.py
```

A few screenshots would also make the `README.md` more pleasant in order to see what users would expect to see. It would help a lot new users (like me) who wish to have a quick overview of TraitsUI capabilities.

Notice that I expressly specified `ETS_TOOLKIT="qt4"` because with the default backend, applications do not show up when clicking on the `Demo` thumbnail.