Add a note not to worry

When reading through the code for the first time and seeing redis,
those unfamiliar with docker first thought might be that they need
to install redis. Adding this sentence helps make it clearer that
this is not needed. Docker will take care of this in services.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

docs/index.md

@@ -75,7 +75,7 @@ Next, you'll want to make a directory for the project:
     $ cd composetest
 
 Inside this directory, create `app.py`, a simple web app that uses the Flask
-framework and increments a value in Redis:
+framework and increments a value in Redis. Don't worry if you don't have Redis installed, docker is going to take care of that for you when we [define services](#define-services):
 
     from flask import Flask
     from redis import Redis