Allow to set custom django settings

Custom settings may be set via the following pillar
structure:

  horizon:
    server:
      django_settings:
        SETTING_NAME:
          enabled: true
          value: 'SETTING_VALUE'

Change-Id: I3f93d02f46143c298b3a2a5ae600beef921d4712
Related-Prod: PROD-25435
diff --git a/README.rst b/README.rst
index f9ead07..fea13b7 100644
--- a/README.rst
+++ b/README.rst
@@ -585,6 +585,20 @@
       server:
         images_allow_location: True
 
+Custom django settings.
+Django has a tonn of usefull settings that might be tuned for particular use case.
+Cover them all in templated manner is not possible. This sections shows how to
+configure custom django setting via horizon metadata.
+
+.. code-block:: yaml
+
+    horizon:
+      server:
+        django_settings:
+          CUSTOM_DJANGO_OPTION:
+            enabled: true
+            value: 'value'
+
 
 Upgrades
 ========