Document scope of macOS/Windows in website?

[chrisglein]

Tom created this write up to help answer FAQs on the macOS repo: https://github.com/microsoft/react-native-macos/wiki/Scope-of-react-native-macos

1. Should we promote that on the website somewhere?
2. Is there anything equivalently we want to do for Windows? (or align existing documentation to make easier to find)