Add installation information to README

Author: akrabat

README.md

@@ -2,6 +2,12 @@
 
 This is a renderer for rendering PHP view scripts into a PSR-7 Response object. It works well with Slim Framework 3.
 
+## Installation
+
+Install with [Composer](http://getcomposer.org):
+
+    composer require slim/php-view
+
 ## Usage With Slim 3
 
 ```php
@@ -32,4 +38,4 @@ $response = $phpView->render(new Response(), "/path/to/template.php", $yourData)
 ## Exceptions
 `\RuntimeException` - if template does not exist
 
-`\InvalidArgumentException` - if $data contains 'template'
+`\InvalidArgumentException` - if $data contains 'template'