Fix #511 by improving README.md

[mgarstenauer]

Fixes #511

Users often forget to add the AvaloniaEdit styles in App.xaml. This step is mentioned in the README.md, but can easily be missed. I know because it happened to me as well.

Users often just look for source code examples that they can copy-paste. Therefore, I suggest to add an example for App.xaml and make it the first example in README.md.