Readme example requires some tweaks

The README example does not work gracefully in an interactive session.  I propose two small changes:

Change the configuration line to 

``` c#
Configuration.Default.Username = "YOUR_API_KEY_HERE"; // this key works for test documents
```

Add `File.WriteAllBytes("test.pdf", create_response);` after the call to CreateDoc, to demonstrate an actual PDF being written to an actual file.
