Add release notes

Ix.NET/Documentation/ReleaseHistory/Ix.v6.md

@@ -1,6 +1,16 @@
 # Ix Release History v6.0
 
 
+## v6.1.0
+
+New functionality:
+
+* Added `ToImmutable*Async` extension methods so `System.Linq.Async` (PR https://github.com/dotnet/reactive/pull/1545 from [Tau Gärtli](https://github.com/bash))
+
+Bug fixes:
+
+ * Fix [Union dispose bug](https://github.com/dotnet/reactive/issues/2112) in `System.Linq.Async`
+
 ## v6.0.1
 
 First release with version number updated to v6.0.x. (At the time, Rx and Ix were attempting to follow a policy of keeping version numbers aligned with the .NET runtime libraries.)