Update installation instruction in README.

Author: weynhamz

README.md

@@ -15,6 +15,16 @@ functionality has been included into an addon plugin for tagbar as
 
 Enjoy!
 
+Installation
+------------
+
+We use [composer](http://getcomposer.org/) for dependency management, run the
+following commands under the project directory to get composer and install the
+dependency.
+
+    curl -s http://getcomposer.org/installer | php
+    php composer.phar install
+
 Requirements
 ------------