[Feature Request] Document Asynchronous Activity Completion in README

[send2vinnie]

Allow the calling of Activity to return without the Activity Execution to complete. The Temporal Client can then be used to both Heartbeat Activity Execution progress and eventually provide complete the Activity Execution and provide a result.

[cretz]

Asynchronous activity is already supported. You can throw new Temporalio.Activities.CompleteAsyncException() to tell the activity you'll be completing outside of the function, and use myClient.GetAsyncActivityHandle(...) to get a handle to manually heartbeat, complete, etc. The task token for use in that call is available via Temporalio.Activities.ActivityExecutionContext.Current.Info.TaskToken.

I will update the title of this issue to just make sure we document this in the README.