Work towards #56: Splitting up the README

I did the job and splitted up the readme, hopefully everything was
splitted correctly...

memento/index.md

@@ -0,0 +1,21 @@
+---
+layout: pattern
+title: Memento
+folder: memento
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Without violating encapsulation, capture and externalize an
+object's internal state so that the object can be restored to this state later.
+
+![alt text](./etc/memento.png "Memento")
+
+**Applicability:** Use the Memento pattern when
+
+* a snapshot of an object's state must be saved so that it can be restored to that state later, and
+* a direct interface to obtaining the state would expose implementation details and break the object's encapsulation
+
+**Real world examples:**
+
+* [java.util.Date](http://docs.oracle.com/javase/8/docs/api/java/util/Date.html)