Update README.md

Author: carloswph

README.md

@@ -2,6 +2,9 @@
 [![PHP Lint](https://img.shields.io/badge/PHP%20Lint-passed-green)](https://github.com/carloswph/ibgecode-php/)<br>
 Simple library to retrieve Brazilian city and respective state from the IBGE code.<br>
 
+# Installation
+Just require the library using Composer `composer require carloswph/ibgecode-php`.
+
 # Usage
 Simple autoload and instantiate the IBGE classe - including an array of the searched codes as parameter. From the initial instance, the class admits methods to get the respective city, the state or both. All methods return and array of results.