add back ultrajson

Author: vinta

README.md

@@ -444,6 +444,7 @@ _Libraries for serializing complex data types._
 - [marshmallow](https://github.com/marshmallow-code/marshmallow) - A lightweight library for converting complex objects to and from simple Python datatypes.
 - [msgpack](https://github.com/msgpack/msgpack-python) - MessagePack serializer implementation for Python.
 - [orjson](https://github.com/ijl/orjson) - Fast, correct JSON library.
+- [ultrajson](https://github.com/esnme/ultrajson) - A fast JSON decoder and encoder written in C with Python bindings.
 
 **Data & Science**