added composer basic docs

docs/guide/composer.md

+Composer
+========
+
+Yii2 uses Composer as its package manager. It is a PHP utility that allows you to automatically install libraries and
+extensions keeping them up to date and handling dependencies.
+
+Installing Composer
+-------------------
+
+Check the official guide for [linux](http://getcomposer.org/doc/00-intro.md#installation-nix) or
+[Windows](http://getcomposer.org/doc/00-intro.md#installation-windows).
+
+Adding more packages to your project
+------------------------------------
+
+After [installing an application](installing.md) you will find `composer.json` in the root directory of your project.
+This file lists packages that your application uses. The part we're interested in is `require` section.
+
+```
+{
+    "require": {
+        "Michelf/php-markdown": ">=1.3",
+        "ezyang/htmlpurifier": ">=4.5.0"
+    }
+}
+```
+
+Here you can specify package name and version. Additionally to Yii extensions you may check
+[packagist](http://packagist.org/) repository for general purpose PHP packages.
+
+After packages are specified you can type either
+
+```
+php composer.phar install
+```
+
+or
+
+```
+php composer.phar update
+```
+
+depending if you're doing it for the first time or not. Then, after some waiting, packages will be installed and ready
+to use. You don't need anything to be configured additionally.
+
+
+See also
+--------
+
+- [Official Composer documentation](http://getcomposer.org).
\ No newline at end of file

docs/guide/index.md

@@ -77,4 +77,5 @@ More
 - [Performance Tuning](performance.md)
 - [Managing assets](assets.md)
 - [Testing](testing.md)
+- [Composer](composer.md)
 - [Upgrading from 1.1 to 2.0](upgrade-from-v1.md)