add docs

README.md

@@ -55,6 +55,9 @@ return [
 ];
 ```
 
+> Note: please refer to the [Symfony Mailer Manual](https://symfony.com/doc/current/mailer.html#transport-setup) for the instructions
+  regarding the transport DSN specification.
+
 Usage example:
 
 ```php
@@ -75,6 +78,8 @@ Yii::app()->mailer->send($email);
 
 ### Configuring Emails Globally <span id="configuring-emails-globally"></span>
 
+In order to simplify the code and avoid its duplication, you can set the default headers, which should be applied for
+each sending email.
 Application configuration example:
 
 ```php
@@ -99,6 +104,10 @@ return [
 
 ### Template rendering <span id="template-rendering"></span>
 
+You may specify the content for the email body to be a result of rendering a particular view templates.
+It works in the similar way as rendering the views in your Web controllers.
+You'll need to use `\yii1tech\mailer\TemplatedEmail` in order to specify the templates.
+
 ```php
 <?php
 
@@ -108,18 +117,45 @@ $email = (new TemplatedEmail())
     ->addFrom('[email protected]')
     ->addTo('[email protected]')
     ->subject('Greetings')
-    ->textTemplate('greetings-text')
-    ->htmlTemplate('greetings-html')
+    ->textTemplate('greetings-text') // Text Body will be set as render of 'views/mail/greetings-text.php'
+    ->htmlTemplate('greetings-html') // HTML Body will be set as render of 'views/mail/greetings-html.php'
     ->context([
-        'name' => 'John Doe',
+        'name' => 'John Doe', // variables, which will be available inside the templates
     ]);
 
-Yii::app()->mailer->send($email);
+Yii::app()->mailer->send($email); // actual rendering takes place just before the sending
+```
+
+By default, the view templates will be searched in the directory "views/mail" in the application base path.
+But you may configure `\yii1tech\mailer\Mailer::$view` component to use a different folder.
+You can also set a global layout for all template rendering.
+Application configuration example:
+
+```php
+<?php
+
+return [
+    'components' => [
+        'mailer' => [
+            'class' => yii1tech\mailer\Mailer,
+            'view' => [
+                'viewPath' => dirname(__DIR__) . '/views/mail',
+                'layout' => 'default-layout',
+            ],
+            // ...
+        ],
+    ],
+    // ...
+];
 ```
 
 
 ### Writing unit tests <span id="writing-unit-tests"></span>
 
+You can use `\yii1tech\mailer\transport\ArrayTransport` while writing unit tests for your application.
+This transport will store all incoming email messages inside the internal field instead of actual sending them.
+Application configuration example:
+
 ```php
 <?php
 
@@ -135,21 +171,35 @@ return [
 ];
 ```
 
+Unit test example:
+
 ```php
 <?php
 
+use Symfony\Component\Mime\Email;
+
 class MailTest extends TestCase
 {
     public function testMail(): void
     {
         // code under test here
+
+        /** @var \yii1tech\mailer\transport\ArrayTransport $transport */
+        $transport = Yii::app()->mailer->getTransport();
 
-        /** @var \Symfony\Component\Mime\Email $sentMessage */
-        $sentMessage = Yii::app()->mailer->getTransport()->getLastSentMessage();
+        // retrieve the last sent email message:
+        $sentMessage = $transport->getLastSentMessage();
 
         $this->assetNotNull($sentMessage);
         $this->assertSame('[email protected]', $sentMessage->getTo()[0]->getAddress());
         // ...
+
+        // retrieve the all sent email messages. matching the callback condition:
+        $messages = $transport->filterSentMessages(function (Email $message) {
+            return $message->getSubject() === 'Greetings';
+        });
+        $this->assertCount(1, $messages);
+        // ...
     }
 }
 ```