Update README to inform users of redis-sentinel's lazy loading behaviour

This PR is basically to inform users that it is actually ok to see a #<Redis client v3.1.0 for redis://localhost:6379/0> after initializing a Redis object. This is because redis-sentinel lazy-loads

While trying to setup redis-sentinel for myself, I kept seeing #<Redis client v3.1.0 for redis://localhost:6379/0> while trying to initialize a redis object in Rails Conosle. I only managed to figure out that redis-sentinel lazy loads after digging into the source code with a friend.