Add a changelog note

There is one (hopefully minor) breaking change and two informative (no
change in behaviour) notes.

Author: e10e3

docs/releases/unreleased.md

@@ -1 +1,7 @@
 # Unreleased
+
+## base
+
+- The `tags` and `more_tags` properties of `base.Estimator` are now both a set of strings.
+- The `base` module is now fully type-annotated. Some type hints have changed, but this does not impact the behaviour of the code. For instance, the regression target is now indicated as a float instead of a Number.
+- `base.Ensemble`, `base.Wrapper`, and `base.WrapperEnsemble` became generic with regard to the type they encapsulate.