Add Installation section

Author: yosmanyga

README.md

@@ -6,6 +6,11 @@ A [Mustache](https://mustache.github.io/) implementation in PHP.
 [![Package version](http://img.shields.io/packagist/v/mustache/mustache.svg?style=flat-square)](https://packagist.org/packages/mustache/mustache)
 [![Monthly downloads](http://img.shields.io/packagist/dm/mustache/mustache.svg?style=flat-square)](https://packagist.org/packages/mustache/mustache)
 
+Installation
+------------
+```
+composer require mustache/mustache
+```
 
 Usage
 -----