Add info on how to install the library

Author: sibprogrammer

README.md

@@ -2,6 +2,12 @@
 
 PHP object-oriented library for Plesk API-RPC.
 
+## Install Via Composer
+
+[Composer](https://getcomposer.org/) is a preferable way to install the library:
+
+`composer require plesk/api-php-lib:@dev`
+
 ## How to Run Unit Tests
 
 One the possible ways to become familiar with the library is to check the unit tests.