Highlight how to retrieve server generated key

It took me an unnecessary amount of time to figure out something as trivial as retrieving this newly generated key from the result object (there was nothing in your documentation: why is there nothing on POST method?!) and only did so by manually cURLing and looking at the response headers.

The PR doesn't fit inline with your existing README's style so I suggest a further edit but the point remains the same: add this to your docs! :)