Work towards #56: Splitting up the README

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

decorator/index.md

@@ -0,0 +1,19 @@
+---
+layout: pattern
+title: Decorator
+folder: decorator
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Attach additional responsibilities to an object dynamically.
+Decorators provide a flexible alternative to subclassing for extending
+functionality.
+
+![alt text](./etc/decorator_1.png "Decorator")
+
+**Applicability:** Use Decorator
+
+* to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects
+* for responsibilities that can be withdrawn
+* when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing