Update README.md

Author: BenGriffiths

README.md

@@ -1,3 +1,5 @@
+## Overview
+
 PHP Remove Unused CSS is a tool to remove unused CSS from your website using PHP. Developed by Momentum 81 - https://momentum81.com.
 
 ```diff
@@ -8,6 +10,14 @@ The main idea is to first compile your CSS into as few files as possible, then y
 
 Often this is done with JS however that can raise issues if you want to work this into a pure PHP development flow.
 
+## Installation
+
+Installation via composer:
+
+```
+composer require momentum81/php-remove-unused-css
+```
+
 ## Example
 
 ``` php