Added README.md for AsyncWorker.

Author: mjohnson12

Samples/AsyncWorker/README.md

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+# AsyncWorker
+
+Demonstrates how to create a `WorkflowConcurrency` async `Worker`.
+
+This is an example of how you would create a closure based network request from within the async function of a `WorkflowConcurrency` `Worker`.
+
+`AsyncWorkerWorkflow.swift` contains the `Worker` implementation.