Add information to the readme explaining what MODEL_REGISTRY, PIPELINE_REGISTRY, etc all do

[luke-carlson]

The configuration system stores a global mapping from name to classes, we should explain how this works in the readme

[pedroborgescruz]

I can work on this!

[aaliyahnl]

https://github.com/apple/ml-mdm/pull/51

[aaliyahnl]

I noticed a potential issue with PIPELINE_CONFIG_REGISTRY on line 36. It seems to store the same information as PIPELINE_REGISTRY. Is this intentional?