Explain outlier detection models

docs/src/adding_models_for_general_use.md

@@ -1193,6 +1193,55 @@ similar fashion. The main differences are:
   input features into a space of lower-dimension. See [Transformers
   that also predict](@ref) for an example.
 
+## Outlier detection models
+
+!!! warning "Experimental API"
+
+    The Outlier Detection API is experimental and may change in future
+    releases of MLJ.
+
+Outlier detection or *anomaly detection* is predominantly an unsupervised
+learning task, transforming each data point to an outlier score quantifying
+the level of "outlierness". However, because detectors can also be
+semi-supervised or supervised, MLJModelInterface provides a collection of
+abstract model types, that enable the different characteristics, namely:
+
+- `MLJModelInterface.SupervisedDetector`
+- `MLJModelInterface.UnsupervisedDetector`
+- `MLJModelInterface.ProbabilisticSupervisedDetector`
+- `MLJModelInterface.ProbabilisticUnsupervisedDetector`
+- `MLJModelInterface.DeterministicSupervisedDetector`
+- `MLJModelInterface.DeterministicUnsupervisedDetector`
+
+All outlier detection models subtyping from any of the above supertypes
+have to implement `MLJModelInterface.fit(model, verbosity, X, [y])`.
+Models subtyping from either `SupervisedDetector` or `UnsupervisedDetector`
+have to implement `MLJModelInterface.transform(model, fitresult, Xnew)`, which
+should return the raw outlier scores (`<:Continuous`) of all points in `Xnew`.
+
+Probabilistic and deterministic outlier detection models provide an additional
+option to predict a normalized estimate of outlierness or a concrete
+outlier label and thus enable evaluation of those models. All corresponding 
+supertypes have to implement (in addition to the previously described `fit`
+and `transform`) `MLJModelInterface.predict(model, fitresult, Xnew)`, with
+deterministic predictions conforming to `OrderedFactor{2}`, with the first
+class being the normal class and the second class being the outlier.
+Probabilistic models predict a `UnivariateFinite` estimate of those classes.
+
+It is typically possible to automatically convert an outlier detection model
+to a probabilistic or deterministic model if the training scores are stored in
+the model's `report`. Below mentioned `OutlierDetection.jl` package, for example,
+stores the training scores under the `scores` key in the `report` returned from
+`fit`. It is then possible to use model wrappers such as 
+`OutlierDetection.ProbabilisticDetector` to automatically convert a model to
+enable predictions of the required output type.
+
+!!! note "External outlier detection packages"
+
+    [OutlierDetection.jl](https://github.com/OutlierDetectionJL/OutlierDetection.jl)
+    provides an opinionated interface on top of MLJ for outlier detection models, 
+    standardizing things like class names, dealing with training scores, score 
+    normalization and more.
 
 ## Convenience methods