update readme

README.md

@@ -25,6 +25,23 @@ There are many breaking changes here; a full set of changenotes is on the way.
 
 ## System.Reactive
 
+### v5.0 changes
+Rx.NET v5 is required for .NET 5 support of CSWinRT; earlier versions of Rx won't work correctly. To get the Windows API, you need to use the latest Windows SDK (19041), though you can target earlier versions of Windows. 
+
+**Breaking changes**
+In Windows, since the `net5.0-windows10.0.19041` target now supports all Windows desktop object models, on UWP `DispatcherObservable` and releated SubscribeOn/ObserveOn were renamed to be `CoreDispatcherObservable`, `SubscribeOnCoreDispatcher`, and `ObserveOnCoreDispatcher`. This reflects the OS type names. `Dispatcher` refers only to the WPF type.
+
+#### Supported Platforms
+Rx 5.x supports the following platforms
+
+- .NET 5
+- .NET 5 with the Windows 10 19041 SDK (able to target earlier Windows versions)
+- .NET Framework 4.6
+- UWP 10.0.16299
+- .NET Standard 2.0
+
+Support for UWP 10.0 has been removed.
+
 ### v4.0 changes
 Due to the [overwhelming](https://github.com/dotnet/reactive/issues/299) [pain](https://github.com/dotnet/reactive/issues/305) that fixing [#205 - Implement assembly version strategy](https://github.com/dotnet/reactive/issues/205) caused, we have refactored the libraries into a single library `System.Reactive`. To prevent breaking existing code that references the v3 libraries, we have facades with TypeForwarders to the new assembly. If you have a reference to a binary built against v3.0, then use the new `System.Reactive.Compatibility` package.