add entry whats new

Author: glemaitre

doc/whats_new/v0.7.rst

@@ -90,6 +90,10 @@ Enhancements
   classification.
   :pr:`780` by :user:`Aurélien Massiot <AurelienMassiot>`.
 
+- Add the class :class:`imblearn.metrics.pairwise.ValueDifferenceMetric` to
+  compute pairwise distances between samples containing only nominal values.
+  :pr:`796` by :user:`Guillaume Lemaitre <glemaitre>`.
+
 Deprecation
 ...........