di doc WIP [skip ci]

docs/guide/README.md

@@ -4,5 +4,6 @@ To add new guide, take the following steps:
 
 1. Create `guide-name` and put there relevant documentation;
 2. If guide has more then one word in name, then it should be with dashes, like: `console-fixture.md`, `module-debug.md`;
-3. If your guide is for console commands, than its name should follow convention: `console-{command}.md`;
-4. If your guide is for custom modules, than its name should follow convention: `module-{moduleName}.md`.
+3. If your guide is about console commands, than its name should follow convention: `console-{command}.md`;
+4. If your guide is about module usages, than its name should follow convention: `module-{moduleName}.md`.
+5. If your guide is about widget usages, than its name should follow convention: `widget-{moduleName}.md`.

docs/guide/di.md

+Service Locator and Dependency Injection
+========================================
+
+Both service locator and dependency injection are design patterns that allow building software
+in a loosely-coupled fashion. Yii uses service locator and dependency injection extensively,
+even though you may not be aware of them. In this tutorial, we will explore their implementation
+and support in Yii to help you write code more consciously. We also highly recommend you to
+read [Martin's article](http://martinfowler.com/articles/injection.html) to get a deeper
+understanding of SL and DI.
+
+
+Service Locator
+---------------
+
+A service locator is an object that knows how to provide all sorts of services that an application
+might need. The most commonly used service locator in Yii is the *application* object accessible through
+`\Yii::$app`. It provides services under the name of *application components*. The following code
+shows how you can obtain an application component (service) from the application object:
+
+```php
+$request = \Yii::$app->get('request');
+// or alternatively
+$request = \Yii::$app->request;
+```
+
+Behind the scene, the application object serves as a service locator because it extends from
+the [[yii\di\ServiceLocator]] class.
+
+
+Dependency Injection
+--------------------

docs/guide/index.md

@@ -36,6 +36,7 @@ Base concepts
 - [Event Handling](events.md) - The Yii event handling mechanism
 - [Behaviors](behaviors.md)
 
+
 Database
 --------
 
@@ -90,6 +91,7 @@ Advanced Topics
 - [Console Application](console.md)
 - [Performance Tuning](performance.md)
 - [Testing](testing.md)
+- [Service Locator and Dependency Injection](di.md)
 
 References
 ----------