Added "How to create an event subscriber"

I've added the information about how to create an event subscriber to the "How to create an Event listener" cookbook recipe. I think that there isn't a recipe in the cookbook about that and it can be useful.
It's because I only wanted to show how to define an event subscriber, and so, show that in an event subscriber we can define various methods that listen to the same event, but I thought that the code of the method wasn't relevant for the example. I added the code of one method to show the similarity between the method in the event listener and the same functionality in the event subscriber one. Therefore, I can remove the code of the method or add some sample code to the other ones. 

Which do you think is the best option?