Update README.md

Author: ogrisel

README.md

@@ -2,6 +2,13 @@
 
 ⚠️⚠️⚠️ The **`sklearn` PyPI package is deprecated use `scikit-learn` instead** ⚠️⚠️⚠️
 
+Note that the `scikit-learn` PyPI package was always the official PyPI package from
+the start of the project: this does not change.
+
+What changes is that the `sklearn` PyPI package which was an empty package with a
+depdendency to `scikit-learn` progressively becomes a package that fails to install
+instead. See this end of this page to see the reasons of this change.
+
 # How to fix the error for the main use cases
 
 - use `pip install scikit-learn` rather than `pip install sklearn`