Rename index.md to README.md

to conform to our standards, every other file is named README.md

data-mapper/README.md

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+---
+layout: pattern
+title: Data Mapper
+folder: data-mapper
+permalink: /patterns/data-mapper/
+categories: Persistence Tier
+tags:
+ - Java
+ - Difficulty-Beginner
+---
+
+## Intent
+A layer of mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself
+
+![alt text](./etc/data-mapper.png "Data Mapper")
+
+## Applicability
+Use the Data Mapper in any of the following situations
+
+* when you want to decouple data objects from DB access layer 
+* when you want to write multiple data retrieval/persistence implementations
+
+## Credits
+
+* [Data Mapper](http://richard.jp.leguen.ca/tutoring/soen343-f2010/tutorials/implementing-data-mapper/)