Adding link to the language proposal.

Ix.NET/Source/System.Interactive.Async/IAsyncDisposable.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
+// See https://github.com/dotnet/csharplang/blob/master/proposals/async-streams.md for the definition of this interface
+// and the design rationale. (8/30/2017)
+
 using System.Threading.Tasks;
 
 namespace System

Ix.NET/Source/System.Interactive.Async/IAsyncEnumerable.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
+// See https://github.com/dotnet/csharplang/blob/master/proposals/async-streams.md for the definition of this interface
+// and the design rationale. (8/30/2017)
+
 namespace System.Collections.Generic
 {
     /// <summary>

Ix.NET/Source/System.Interactive.Async/IAsyncEnumerator.cs

@@ -2,6 +2,9 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
+// See https://github.com/dotnet/csharplang/blob/master/proposals/async-streams.md for the definition of this interface
+// and the design rationale. (8/30/2017)
+
 using System.Threading.Tasks;
 
 namespace System.Collections.Generic