Work towards #56: Splitting up the README

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

mediator/index.md

@@ -0,0 +1,19 @@
+---
+layout: pattern
+title: Mediator
+folder: mediator
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Define an object that encapsulates how a set of objects interact.
+Mediator promotes loose coupling by keeping objects from referring to each
+other explicitly, and it lets you vary their interaction independently.
+
+![alt text](./etc/mediator_1.png "Mediator")
+
+**Applicability:** Use the Mediator pattern when
+
+* a set of objects communicate in well-defined but complex ways. The resulting interdependencies are unstructured and difficult to understand
+* reusing an object is difficult because it refers to and communicates with many other objects
+* a behavior that's distributed between several classes should be customizable without a lot of subclassing