Work on #226, moved POSA reference and some J2EE design pattern references

dao/index.md

@@ -4,7 +4,9 @@ title: Data Access Object
 folder: dao
 permalink: /patterns/dao/
 categories: Architectural
-tags: Java
+tags:
+ - Java
+ - Difficulty-Beginner
 ---
 
 **Intent:** Object provides an abstract interface to some type of database or
@@ -16,3 +18,7 @@ other persistence mechanism.
 
 * when you want to consolidate how the data layer is accessed
 * when you want to avoid writing multiple data retrieval/persistence layers
+
+**Credits:**
+
+* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)