Document breaking change for #46044 (#51070)

Author: cston

docs/compilers/CSharp/Compiler Breaking Changes - post DotNet 5.md

@@ -0,0 +1,13 @@
+## This document lists known breaking changes in Roslyn after .NET 5.
+
+1. https://github.com/dotnet/roslyn/issues/46044 In C# 9.0 (Visual Studio 16.9), a warning is reported when assigning `default` to, or when casting a possibly `null` value to a type parameter type that is not constrained to value types or reference types. To avoid the warning, the type can be annotated with `?`.
+    ```C#
+    static void F1<T>(object? obj)
+    {
+        T t1 = default; // warning CS8600: Converting possible null value to non-nullable type
+        t1 = (T)obj;    // warning CS8600: Converting possible null value to non-nullable type
+
+        T? t2 = default; // ok
+        t2 = (T?)obj;    // ok
+    }
+    ```