Update README.md: multi-part upload task example

I found the documentation lacking an example of how to do multi-part uploads with feedback through NSProgress.
Specifically the crucial connection between multi-part upload and `uploadTaskWithStreamedRequest:progress:completionHandler:`.

However, as it was not working correctly until https://github.com/AFNetworking/AFNetworking/pull/1617 this change could probably be included at the same time as that change is "released".
Merge pull request #1742 from johanforssell/patch-1

Update README.md: multi-part upload task example