Work towards #56: Splitting up the README

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

callback/index.md

@@ -0,0 +1,21 @@
+---
+layout: pattern
+title: Callback
+folder: callback
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Callback is a piece of executable code that is passed as an
+argument to other code, which is expected to call back (execute) the argument
+at some convenient time.
+
+![alt text](./etc/callback.png "Callback")
+
+**Applicability:** Use the Callback pattern when
+
+* when some arbitrary synchronous or asynchronous action must be performed after execution of some defined activity.
+
+**Real world examples:**
+
+* [CyclicBarrier] (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept callback that will be triggered every time when barrier is tripped.