Adding explanation about variables scope and naming convention

As I read the text, I saw that there is no information about how Ruby relies on the way that we name variables to define the scope of that variable. And also there is no information about constants, that are in every single class definition.

One thing to note is that I'm not sure if the place that I put this information is the best one. I placed it right before the explanation on classes because this way the reader will at least know how classes are in Ruby and then can understand the difference between instance/class variables.

Please, let me know if you can think of a better place to put that, if there is something else that could be added or if it just don't fit in learnxinyminutes.