Document how to turn existing Docker images into Cog images

[andreasjansson]

A lot of people have working Docker images already, so they might not want re-package their models with Cog. In order to use Replicate and other systems that run Cog models, we should document how to turn an existing image into a Cog model.

It should be as easy as installing the Cog Python package and implementing cog.Predictor. That doesn't work yet since we don't continuously publish Cog to PyPI.

[Quint-e]

Just adding a reference to a similar "Bring Your Own Container" concept in AWS SageMaker: https://docs.aws.amazon.com/sagemaker/latest/dg/your-algorithms-training-algo-dockerfile.html