Evaluate recommendations of islands in documentation

[asklar]

https://github.com/microsoft/react-native-windows-samples/blob/master/samples/TodosFeed/README.md

specifically:
• As a component, that can be used in a WPF application through XAML Islands
We discussed a few weeks ago that we don’t want to advertise Win32 or islands, so we shouldn’t have this documented or if we mention it, we should say it is unsupported.
Related: Rationalize public win32/island support #267

Thoughts?

[chrisglein]

Need to highlight the difference between golden path (UWP) and places where it is possible to connect other tech together but is either less recommended or may have some gaps that we need to highlight.