#13 Add documentation about use Redis without have the module enabled

[guiajlopes]

Adding documentation to cover #13

[andypost]

+1 to get this in

[rodrigoaguilera]

This a great piece of information +1 to add it to the docs

[lisotton]

I think should include also in this documentation that in this case the content of redis.services.yml should be copied to sites/default/services.yml, otherwise you will get:

You have requested a non-existent service "cache.backend.redis". Did you mean one of these: "cache.backend.apcu",  
   "cache.backend.php", "cache.backend.memory", "cache.backend.null"?

[lisotton]

Also, I needed to use double back-slash and add it to the end, otherwise I was getting:

A non-empty PSR-4 prefix must end with a namespace separator.

So my suggestion is:

$class_loader->addPsr4('Drupal\\redis\\', DRUPAL_ROOT . '/[redis_folder]/src');