Add Maxnet.jl to the model registry

[tiemvanderdeure]

I implemented a MaxnetBinaryClassifier in my package Maxnet.jl. Can it be added to the registry?

I'm happy to make some tweaks if needed.

[ablaom]

Thanks for working on an MLJ interface! Any chance you could add an interface test, as here? You'll need MLJTestInterface in your [extras].

[ablaom]

Also, if you haven't already, an MLJ-compliant doc string which preferably includes a self-contained example. (Also helpful for our review).