Add clear documentation to avoid using this repo for predictions

[ngreenwald]

Is your feature request related to a problem? Please describe.
It seems that because this repo has the name Nimbus, if people just search for the github rather than following the link from the paper, they may think this is the one they're supposed to be using.

Describe the solution you'd like
Update the readme to clearly indicate that if all people want to do is run the trained model, they should instead use the inference repo. We should probably add the same thing to the nimbus prediction notebook in this repo, or maybe remove it.