Added a user guide based on MkDocs

doc/about/see_also.md

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+# See also
+
+## Development
+- [API reference](https://dev.sabcomputer.com/gitea.php/api)
+- [Packagist package](https://packagist.org/packages/sab-international/gitea)
+- [Submit an issue](https://github.com/sab-international/gitea.php/issues)
+
+## Testing
+- [Continuous integration](https://travis-ci.com/sab-international/gitea.php)
+- [Code coverage](https://coveralls.io/github/sab-international/gitea.php/)
+
+## Other implementations
+- Dart: [Gitea for Dart](https://dev.belin.io/gitea.dart)
+- JavaScript: [Gitea for JS](https://dev.belin.io/gitea.js)
+- Yii Framework: [Gitea for Yii](https://dev.sabcomputer.com/yii2-gitea)

doc/index.md

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+# Gitea <small>for PHP</small>
+
+## Yet another implementation of enumerated types
+This implementation, based on [traits](https://secure.php.net/manual/en/language.oop5.traits.php), does not try to reproduce the semantics of traditional enumerations, like the ones found in C# or Java languages.
+
+Unlike other [PHP](https://secure.php.net) implementations, like the [SplEnum](https://secure.php.net/manual/en/class.splgitea.php) class, it does not rely on object instances. Instead, it just gives a set of static methods to ease working with the `public` constants of a class representing an enumerated type.
+
+## Quick start
+Install the latest version of **Enums for PHP** with [Composer](https://getcomposer.org):
+
+```shell
+composer require sab-international/gitea
+```
+
+For detailed instructions, see the [installation guide](installation.md).

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};
+```

doc/usage.md

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+path: src/branch/master
+source: lib/EnumTrait.php
+# Usage
+
+## Create the enumeration
+Just use the `Enum\EnumTrait` trait on a class:
+
+```php
+<?php
+use Enum\{EnumTrait};
+
+/**
+ * Specifies the day of the week.
+ */
+final class DayOfWeek {
+  use EnumTrait;
+
+  const SUNDAY = 0;
+  const MONDAY = 1;
+  const TUESDAY = 2;
+  const WEDNESDAY = 3;
+  const THURSDAY = 4;
+  const FRIDAY = 5;
+  const SATURDAY = 6;
+}
+```
+
+This trait adds a private constructor to the enumerated type: it prohibits its instantiation.
+
+Thus, the obtained enumeration can only contain static members.
+You should only use [scalar constants](https://secure.php.net/manual/en/function.is-scalar.php), and possibly methods.
+
+## Work with the enumeration
+Check whether a value is defined among the enumerated type:
+
+```php
+<?php
+DayOfWeek::isDefined(DayOfWeek::SUNDAY); // true
+DayOfWeek::isDefined('foo'); // false
+```
+
+Ensure that a value is defined among the enumerated type:
+
+```php
+<?php
+DayOfWeek::assert(DayOfWeek::MONDAY); // DayOfWeek::MONDAY
+DayOfWeek::assert('foo'); // (throws \UnexpectedValueException)
+
+DayOfWeek::coerce(DayOfWeek::MONDAY); // DayOfWeek::MONDAY
+DayOfWeek::coerce('bar'); // null
+DayOfWeek::coerce('baz', DayOfWeek::TUESDAY); // DayOfWeek::TUESDAY
+```
+
+Get the zero-based position of a value in the enumerated type declaration:
+
+```php
+<?php
+DayOfWeek::getIndex(DayOfWeek::WEDNESDAY); // 3
+DayOfWeek::getIndex('foo'); // -1
+```
+
+Get the name associated to an enumerated value:
+
+```php
+<?php
+DayOfWeek::getName(DayOfWeek::THURSDAY); // "THURSDAY"
+DayOfWeek::getName('foo'); // "" (empty)
+```
+
+Get information about the enumerated type:
+
+```php
+<?php
+DayOfWeek::getEntries();
+// ["SUNDAY" => 0, "MONDAY" => 1, "TUESDAY" => 2, "WEDNESDAY" => 3, "THURSDAY" => 4, "FRIDAY" => 5, "SATURDAY" => 6]
+
+DayOfWeek::getNames();
+// ["SUNDAY", "MONDAY", "TUESDAY", "WEDNESDAY", "THURSDAY", "FRIDAY", "SATURDAY"]
+
+DayOfWeek::getValues();
+// [0, 1, 2, 3, 4, 5, 6]
+```