Fixed #26974 -- Added HashIndex to django.contrib.postgres.

Thanks Akshesh Doshi for the initial implementation.

docs/releases/2.2.txt

@@ -82,6 +82,9 @@ Minor features
   :class:`~django.contrib.postgres.aggregates.StringAgg` determines the
   ordering of the aggregated elements.
 
+* The new :class:`~django.contrib.postgres.indexes.HashIndex` class
+  allows creating ``hash`` indexes in the database.
+
 :mod:`django.contrib.redirects`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~