Add example using asynchronous returns

[rabbah]

Currently I don't think there is a way to return a Future from the .NET action, making it less convenient to write asynchronous functions.

[shawnallen85]

We support async/await methods in the .NET runtime.

https://docs.microsoft.com/en-us/dotnet/csharp/async

The testEchoNoWrite test (in all runtimes) should be calling this method, which is asynchronous.

[rabbah]

Thanks! Indeed this is what I was looking for. We should add this to the docs. The particular use case I ran into was related to making an http request. Retitling.

[shawnallen85]

This'll be resolved here: #50