Add installation instructions

Author: alexeevdv

README.md

@@ -19,6 +19,24 @@ If you have developed software based around phpagi, we'd like to hear from
 you!  Drop us a note, and indicate whether you'd like us to list your 
 application on our website.
 
+## Installation
+
+The preferred way to install this extension is through [composer](https://getcomposer.org/download/).
+
+Either run
+
+```bash
+$ composer require welltime/phpagi ^2.20
+```
+
+or add
+
+```
+"welltime/phpagi": "^2.20"
+```
+
+to the ```require``` section of your `composer.json` file.
+
 FILES
 -----
 * phpagi.php           - The main phpagi class.