Add data mapper

Author: mgechev

README.md

@@ -789,10 +789,13 @@ Once we want to inject `foo` inside any other component we won't be able to use
 
 ## AngularJS application Patterns
 
-### Prototype
-
 ### Data Mapper
 
+>A Data Mapper is a Data Access Layer that performs bidirectional transfer of data between a persistent data store (often a relational database) and an in memory data representation (the domain layer). The goal of the pattern is to keep the in memory representation and the persistent data store independent of each other and the data mapper itself.
+
+![Data Mapper](./images/data-mapper.png "Fig. 10")
+ 
+
 ## References
 
 1. [Wikipedia](https://en.wikipedia.org/wiki). The source of all brief descriptions of the design patterns is wikipedia.