Add "Low-Level Caching" part to "Caching With Rails" guide

I noticed that most of the people that are working with Rails (including me) are not familiar enough with low-level concepts of Rails caching mechanism and treat it only as a mechanism to cache view data. So I decided to add some useful info, and used a part of an [article by Heroku](https://devcenter.heroku.com/articles/caching-strategies#further-reading) for this purpose.