Added description of Dependency Injection pattern in README.md.

Author: iluwatar

README.md

@@ -60,6 +60,7 @@ Behavioral patterns are concerned with algorithms and the assignment of responsi
 * [Null Object](#null-object)
 * [Intercepting Filter](#intercepting-filter)
 * [Specification](#specification)
+* [Dependency Injection](#dependency-injection)
 
 ### Concurrency Patterns
 
@@ -629,6 +630,15 @@ validation and for building to order
 * The objects are expensive to create (allocation cost)
 * You need a large number of short-lived objects (memory fragmentation)
 
+## <a name="dependency-injection">Dependency Injection</a> [&#8593;](#list-of-design-patterns)
+**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](https://github.com/iluwatar/java-design-patterns/blob/master/dependency-injection/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
+
 
 
 # Frequently asked questions