Add Redis component to README.rst

Redis infra component has been introduced during commit
e3423bea2f.

Change-Id: I26395fea6e2f888478d9363693a68ca7905d6381

README.rst

@@ -108,6 +108,7 @@ Kolla-Ansible deploys containers for the following infrastructure components:
 - `Open vSwitch <http://openvswitch.org/>`__ and Linuxbridge backends for Neutron.
 - `RabbitMQ <https://www.rabbitmq.com/>`__ as a messaging backend for
   communication between services.
+- `Redis <https://redis.io/>`__ an in-memory data structure store.
 
 Directories
 ===========