Additions to usage scenario

hi, 

I'd like to contribute the following text to the usage document:

<text>
2.21.3 Requirements

  2. A receiver should be able to return an immediate receipt, which
contains the following:
    1. A URI to the result. The URI will be later bound to the result when
it becomes available.
    2. A URI to the status of the current request. A client will be able to
poll the status URI to know its request status.
    3. A timestamp and other business specific information.
</text>
    
The motivation is that this allows a client to pull results rather than
being pushed.     

Hao