Work towards #56: Splitting up the README

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

dependency-injection/index.md

@@ -0,0 +1,21 @@
+---
+layout: pattern
+title: Dependency Injection
+folder: dependency-injection
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Dependency Injection is a software design pattern in which one or
+more dependencies (or services) are injected, or passed by reference, into a
+dependent object (or client) and are made part of the client's state. The
+pattern separates the creation of a client's dependencies from its own
+behavior, which allows program designs to be loosely coupled and to follow the
+inversion of control and single responsibility principles.
+
+![alt text](./etc/dependency-injection.png "Dependency Injection")
+
+**Applicability:** Use the Dependency Injection pattern when
+
+* when you need to remove knowledge of concrete implementation from object
+* to enable unit testing of classes in isolation using mock objects or stubs