Added a user guide based on MkDocs

doc/installation.md

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+# Installation
+
+## Requirements
+Before installing **Enums for PHP**, you need to make sure you have [PHP](https://secure.php.net)
+and [Composer](https://getcomposer.org), the PHP package manager, up and running.
+
+!!! warning
+    Enums for PHP requires PHP >= **7.2.0**.
+    
+You can verify if you're already good to go with the following commands:
+
+```shell
+php --version
+# PHP 7.2.10-0ubuntu0.18.04.1 (cli) (built: Sep 13 2018 13:45:02) ( NTS )
+
+composer --version
+# Composer version 1.7.2 2018-08-16 16:57:12
+```
+
+!!! info
+    If you plan to play with the package sources, you will also need
+    [Robo](https://robo.li) and [Material for MkDocs](https://squidfunk.github.io/mkdocs-material).
+
+## Installing with Composer package manager
+
+### 1. Install it
+From a command prompt, run:
+
+```shell
+composer require sab-international/gitea
+```
+
+### 2. Import it
+Now in your [PHP](https://secure.php.net) code, you can use:
+
+```php
+<?php
+use Enum\{EnumTrait};
+```