Added a clear "what does Redis do".

When I first came across Redis, I was unsure as to why I couldn't just stuff everything in MySQL and then just access from there.

In fact, this StackOverflow question deals with exactly that: http://stackoverflow.com/questions/3966689/when-to-use-redis-instead-of-mysql-for-php-applications

I think this is a common tripup for many newcomers to Redis (I've seen this time and time again on #rubyonrails on Freenode). I think it makes sense to include this in the introduction.