Add entry in whats_new about self.estimators_ disappearing from ensem…

…ble estimators

doc/whats_new.rst

@@ -119,6 +119,16 @@ Bug fixes
      the same result as the LassoLars implementation available
      in R (lars library). :issue:`7849` by `Jair Montoya Martinez`_
 
+API changes summary
+-------------------
+
+   - Ensure that estimators' attributes ending with ``_`` are not set
+     in the constructor but only in the ``fit`` method. Most notably
+     if you implemented your own estimator deriving from
+     :class:`ensemble.BaseEnsemble` you may be affected by this if you
+     were using `self.estimators_` in your constructor.
+
+
 .. _changes_0_18_1:
 
 Version 0.18.1