Work towards #56: Splitting up the README

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

double-dispatch/index.md

@@ -0,0 +1,20 @@
+---
+layout: pattern
+title: Double Dispatch
+folder: double-dispatch
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Double Dispatch pattern is a way to create maintainable dynamic
+behavior based on receiver and parameter types.
+
+![alt text](./etc/double-dispatch.png "Double Dispatch")
+
+**Applicability:** Use the Double Dispatch pattern when
+
+* the dynamic behavior is not defined only based on receiving object's type but also on the receiving method's parameter type.
+
+**Real world examples:** 
+
+* [ObjectOutputStream](https://docs.oracle.com/javase/8/docs/api/java/io/ObjectOutputStream.html)