Update README.md.

Author: CXuesong

README.md

@@ -1,6 +1,8 @@
 # AsyncEnumerableExtensions
 
 >   The package is available on NuGet as `CXuesong.AsyncEnumerableExtensions`.
+>
+>   As of May, 2019, there is `IAsyncEnumerable` on .NET Standard 2.1 / .NET Core 3.0. Thus this package uses built-in `IAsyncEnumerable` instead of the one in `Ix.Async` on these supporting platform. Note that prior platform versions will still target to `Ix.Async`.
 
 Some rudimentary utilities to flavor [`Ix.Async`](https://github.com/Reactive-Extensions/Rx.NET), such as…
 
@@ -26,6 +28,7 @@ public async void NormalGeneratorTest()
         finished = true;
     }
 
+    // Replace ToArray with ToArrayAsync on .NET Core 3.0 / .NET Standard 2.1
     var array = await AsyncEnumerableFactory.FromAsyncGenerator<int>(Generator).ToArray();
     Assert.True(finished);
     Assert.Equal(new[] {10, 20, 30, 40, 50}, array);