Document how to add plot recipes in a new model implementation

[ablaom]

@MilesCranmer Any chance you could add a couple of lines, maybe just pointing to your implementation as an example?

It could go somewhere here - if I don't like where you put it, no matter, I can move it.

[MilesCranmer]

Sure, will do. I need to merge MilesCranmer/SymbolicRegression.jl#277 first as this creates a custom type SRFitResult for the fitresult used by SRRegressor, so that the recipe is assigned to only the fitresults from my regressors. I think this is the "right" way to do things and once its ready I will share the example (ping me if I forget!)

Current example: https://github.com/MilesCranmer/SymbolicRegression.jl/blob/4b6a3cd44f30bfa92e2fe1e53718ca85c61df088/src/PlotRecipes.jl

[MilesCranmer]

By the way, I think the new MLJBase version was not yet released? JuliaAI/MLJBase.jl@193ad67

[ablaom]

? JuliaRegistries/General#98572

[MilesCranmer]

Oh I see it now, thanks!