Added HStoreField.

Thanks to `django-hstore` for inspiration in some areas, and many people
for reviews.

docs/ref/contrib/postgres/forms.txt

@@ -133,3 +133,23 @@ SplitArrayField
             ['1', '2', '']  # -> [1, 2]
             ['1', '', '3']  # -> [1, None, 3]
             ['', '2', '']  # -> [None, 2]
+
+HStoreField
+-----------
+
+.. class:: HStoreField
+
+    A field which accepts JSON encoded data for an
+    :class:`~django.contrib.postgres.fields.HStoreField`. It will cast all the
+    values to strings. It is represented by an HTML ``<textarea>``.
+
+    .. admonition:: User friendly forms
+
+        ``HStoreField`` is not particularly user friendly in most cases,
+        however it is a useful way to format data from a client-side widget for
+        submission to the server.
+
+    .. note::
+        On occasions it may be useful to require or restrict the keys which are
+        valid for a given field. This can be done using the
+        :class:`~django.contrib.postgres.validators.KeysValidator`.