Work towards #56: Splitting up the README

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

observer/index.md

@@ -0,0 +1,27 @@
+---
+layout: pattern
+title: Observer
+folder: observer
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Define a one-to-many dependency between objects so that when one
+object changes state, all its dependents are notified and updated
+automatically.
+
+![alt text](./etc/observer_1.png "Observer")
+
+**Applicability:** Use the Observer pattern in any of the following situations
+
+* when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
+* when a change to one object requires changing others, and you don't know how many objects need to be changed
+* when an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
+
+**Typical Use Case:**
+
+* changing in one object leads to a change in other objects
+
+**Real world examples:**
+
+* [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)