Release notes.

Author: jpobst

Documentation/release-notes/4227.md

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+#### Mono.Android.dll Nullable Reference Type Annotations
+
+`Mono.Android.dll` assemblies of all platform levels are now annotated with 
+C#8's nullable reference types (NRT).  Users who opt their applications into 
+this feature with `<Nullable>enable</Nullable>` will receive warnings if their 
+code does not properly account for possible `null` values.
+
+General documentation for the NRT feature is available here: 
+https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references.
+
+Note:
+The majority of `Mono.Android.dll` is automatically generated from the Android 
+Java source, including these new annotations.  As such, we will not be manually 
+fixing places where the Android source code is not annotated correctly.
+
+If there is an error regarding nullability for any of the Mono.Android APIs 
+that Xamarin adds to the Android source (such as `JavaList` or `InputStreamAdapter`), 
+please file a bug so we can properly annotate our additions.