Mention sortedcontainers-pydantic for Pydantic support

[jayqi]

Hello!

I wanted to use Sorted Containers classes with Pydantic models, so I made a sortedcontainers-pydantic package:

• PyPI: https://pypi.org/project/sortedcontainers-pydantic/
• GitHub: https://github.com/drivendataorg/sortedcontainers-pydantic

It implements the special methods that Pydantic uses to validate/serialize types. I thought it might be helpful to mention it somewhere in this project's documentation, in case others have the same need.