Work towards #56: Splitting up the README

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

private-class-data/index.md

@@ -0,0 +1,17 @@
+---
+layout: pattern
+title: Private Class Data
+folder: private-class-data
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Private Class Data design pattern seeks to reduce exposure of
+attributes by limiting their visibility. It reduces the number of class
+attributes by encapsulating them in single Data object.
+
+![alt text](./etc/private-class-data.png "Private Class Data")
+
+**Applicability:** Use the Private Class Data pattern when
+
+* you want to prevent write access to class data members