Fixes async payload passing for #68.

docs/async.md

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+# Asynchronous Functions
+
+Asynchronous (async) functions will run your function at some point in the future. The default mode is synchronous which means
+a function is executed and the caller blocks while waiting for the response. Asynchronous on the other, puts the request into a 
+message queue and responds immediately to the caller. The function will then be executed at some point in the future, upon resource availability giving priority
+to synchronous calls. Also, since it is using a message queue, you can safely queue up millions of function calls without worrying about
+capacity. 
+
+Async will return immediately with a `call_id`, for example:
+
+```json
+{"call_id": "abc123"}
+```
+
+The `call_id` can then be used to retrieve the status at a later time. 
+
+Asynchronous function calls are great for tasks that are CPU heavy or take more than a few seconds to complete.
+For instance, image processing, video processing, data processing, ETL, etc.