Add explaination to readme

Author: alejandrosobko

README.md

@@ -2,6 +2,27 @@
 
 Common functions implemented using recursion
 
+---
+
+### How is the idea?
+
+I tried to follow the haskell way, which is basically:
+
+- base case
+- recursive case
+
+
+So, almost always we'll have:
+
+- empty list
+- list with elements
+
+(`flatten` is a special case, take a look at the file  😉 )
+
+For empty list, we don't do anything. For the case of the list with elements, we separate into first element and the rest. We _do_ something to the first element, and call the recursion to complete rest. And voila!
+
+
+
 ---
 
 ### Implemented: