Work towards #56: Splitting up the README

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

event-aggregator/index.md

@@ -0,0 +1,24 @@
+---
+layout: pattern
+title: Event Aggregator
+folder: event-aggregator
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** A system with lots of objects can lead to complexities when a
+client wants to subscribe to events. The client has to find and register for
+each object individually, if each object has multiple events then each event
+requires a separate subscription. An Event Aggregator acts as a single source
+of events for many objects. It registers for all the events of the many objects
+allowing clients to register with just the aggregator.
+
+![alt text](./etc/classes.png "Event Aggregator")
+
+**Applicability:** Use the Event Aggregator pattern when
+
+* Event Aggregator is a good choice when you have lots of objects that are
+  potential event sources. Rather than have the observer deal with registering
+  with them all, you can centralize the registration logic to the Event
+  Aggregator. As well as simplifying registration, a Event Aggregator also
+  simplifies the memory management issues in using observers.