WIP on docs and added basic enums supports

docs/00-intro.md

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+# Ulmus
+
+Welcome to the official Ulmus documentation.
+
+## Quick start
+
+Creating a simple user entity:
+
+*/app/entity/user.php*
+```php
+<?php
+
+namespace MyApp\Entity;
+
+use Ulmus\Entity\Field\Datetime;
+
+/**
+ * @Table('name' => "user")
+ */
+class User
+{
+    use \Ulmus\EntityTrait;
+    
+    /**
+    * @Id
+    */
+    public int $id;
+    
+    /**
+    * @Field 
+    */
+    public string $fullname;
+    
+    /**
+    * @Field 
+    */
+    public ? string $email;
+    
+    /**
+    * @Field('name' => 'is_admin')
+    */
+    public bool $isAdmin = false;
+    
+    /**
+    * @DateTime 
+    */
+    public Datetime $birthday;
+}
+```
+
+### Loading a user using a Primary Key
+
+With this entity, we could quickly load a user using the `@Id` field using:
+
+```php
+$user = Entity\User::repository()->loadFromPk((int) $_GET['id']);
+```
+
+Or we could also, load another single entity using a specific field:
+
+```php
+# fetch a single user
+$user = Entity\User::repository()->loadOneFromField((string) $_POST['email'], 'email');
+
+# Fetch every admins
+$admin_list = Entity\User::repository()->loadFromField(true, 'isAdmin');
+```
+
+Using the same entity class, we could create a new user and save it using:
+
+```php
+$user = new Entity\User();
+$user->fullname = "Johnny Does";
+$user->birthday = "1980-01-15";
+
+if ( Entity\User::repository()->save($user) ) {
+    echo "User created successfully !";
+}
+else {
+    echo "User could not be saved :\";
+}
+```
+
+Which would result in the following query (from MySQL adapter) being generated and executed :
+
+```SQL
+INSERT INTO `user` VALUES fullname = :fullname, birthday = :birthday;
+```
+
+Binded using:
+
+```PHP
+'fullname' => "Johnny Does",
+'birthday' => "1980-01-15",
+```