text: Title: Add example for how to extend support for other serialization formats
  Body: This can be done by [writing a custom converter factory](https://uplink.readthedocs.io/en/stable/user/serialization.html#writing-a-custom-converter).  We should add an example to the  [`examples`](https://github.com/prkumar/uplink/tree/master/examples) directory on how to do this. 