add mailer docs

docs/installation/mailer.md

@@ -0,0 +1,58 @@
+Mailer
+======
+
+The way this module sends its emails is throughout the [`Mailer`](http://www.yiiframework.com/doc-2.0/guide-tutorial-mailing.html) 
+component of Yii 2. Please, follow Yii 2's guidelines to set it up. 
+ 
+Nevertheless, you have to configure the following attribute of the module: `mailParams`. the following is its default 
+values:
+
+```php
+[
+    'fromEmail' => 'no-reply@example.com',
+    'welcomeMailSubject' => Yii::t('usuario', 'Welcome to {0}', $app->name),
+    'confirmationMailSubject' => Yii::t('usuario', 'Confirm account on {0}', $app->name),
+    'reconfirmationMailSubject' => Yii::t('usuario', 'Confirm email change on {0}', $app->name),
+    'recoveryMailSubject' => Yii::t('usuario', 'Complete password reset on {0}', $app->name),
+]
+```
+
+Actually, the only thing required is the `fromEmail` value. If you look at the code of `Da\User\Factory\MailFactory.php` 
+you will easily find the reason why: 
+
+```php
+// take this helper function for example: 
+
+public static function makeRecoveryMailerService($email, Token $token = null)
+{
+    /** @var Module $module */
+    $module = Yii::$app->getModule('user');
+    $to = $email;
+    $from = $module->mailParams['fromEmail']; // fromEmail!!!
+    $subject = $module->mailParams['recoveryMailSubject']; // subject!!!
+    $params = [
+        'user' => $token && $token->user ? $token->user : null,
+        'token' => $token,
+    ];
+
+    return static::makeMailerService($from, $to, $subject, 'recovery', $params);
+}
+
+```
+
+With that information it creates an `Da\User\Service\MailService` instance and this class makes use of those values to 
+actually send the mails: 
+
+```php
+public function run()
+{
+    return $this->mailer
+        ->compose(['html' => $this->view, 'text' => "text/{$this->view}"], $this->params)
+        ->setFrom($this->from) // $this->from is actually fromEmail!!!
+        ->setTo($this->to)
+        ->setSubject($this->subject) // $this->subject is actually recoveryMailSubject!!!
+        ->send();
+}
+```
+
+© [2amigos](http://www.2amigos.us/) 2013-2017