Updating Note to Setup guide

Due to [1] patch it's better to update [2] file.
Otherwise it create confusion to a new devloper if use
"tox -e runserver" for developments.

[1] https://review.openstack.org/#/c/630790
[2] openstack_dashboard/local/local_settings.py.example

Change-Id: I2146b33f46c417c5f1d61a4958336c92f0c03e9c

doc/source/contributor/quickstart.rst

@@ -86,7 +86,10 @@ To start the Horizon development server use the command below
     The default port for runserver is 8000 which might be already consumed by
     heat-api-cfn in DevStack. If running in DevStack
     ``tox -e runserver -- localhost:9000`` will start the test server at
-    ``http://localhost:9000``
+    ``http://localhost:9000``. If you use ``tox -e runserver`` for developments,
+    then configure ``SESSION_ENGINE`` to
+    ``django.contrib.sessions.backends.signed_cookies`` in
+    ``openstack_dashboard/local/local_settings.py`` file.
 
 Once the Horizon server is running, point a web browser to ``http://localhost``
 or to the IP and port the server is listening for. Enter your Keystone

openstack_dashboard/local/local_settings.py.example

@@ -171,6 +171,12 @@ SECRET_KEY = secret_key.generate_or_read_from_file(
 #    },
 #}
 
+# If you use ``tox -e runserver`` for developments,then configure
+# SESSION_ENGINE to django.contrib.sessions.backends.signed_cookies
+# as shown below:
+#SESSION_ENGINE = 'django.contrib.sessions.backends.signed_cookies'
+
+
 # Send email to the console by default
 EMAIL_BACKEND = 'django.core.mail.backends.console.EmailBackend'
 # Or send them to /dev/null