Add documentation to add new mail transports

doc/plugin-mail-transports.markdown

@@ -0,0 +1,50 @@
+Plugin: Add Mail Transport
+==========================
+
+By default Kanboard supports 3 standards mail transports:
+
+- Mail (PHP mail function)
+- Smtp
+- Sendmail command
+
+With the plugin API you can add a driver for any email provider.
+By example, your plugin can add a mail transport for a provider that uses an HTTP API.
+
+Implementation
+--------------
+
+Your plugin must implements the interface `Kanboard\Core\Mail\ClientInterface` and extends from `Kanboard\Core\Base`.
+
+The only method you need to implement is `sendEmail()`:
+
+```php
+interface ClientInterface
+{
+    /**
+     * Send a HTML email
+     *
+     * @access public
+     * @param  string  $email
+     * @param  string  $name
+     * @param  string  $subject
+     * @param  string  $html
+     * @param  string  $author
+     */
+    public function sendEmail($email, $name, $subject, $html, $author);
+}
+```
+
+To register your new mail transport, use the method `setTransport($transport, $class)` from the class `Kanboard\Core\Mail\Client`:
+
+```php
+$this->emailClient->setTransport('myprovider', '\Kanboard\Plugin\MyProvider\MyEmailHandler');
+```
+
+The second argument contains the absolute namespace of your concrete class.
+
+Examples of mail transport plugins
+----------------------------------
+
+- [Sendgrid](https://github.com/kanboard/plugin-sendgrid)
+- [Mailgun](https://github.com/kanboard/plugin-mailgun)
+- [Postmark](https://github.com/kanboard/plugin-postmark)

doc/plugins.markdown

@@ -7,10 +7,11 @@ Plugins are useful to extend the core functionalities of Kanboard, adding featur
 
 Plugin creators should specify explicitly the compatible versions of Kanboard. Internal code of Kanboard may change over the time and your plugin must be tested with new versions.
 
-- [Plugin Registration](plugin-registration.markdown)
-- [Plugin Hooks](plugin-hooks.markdown)
-- [Plugin Overrides](plugin-overrides.markdown)
-- [Plugin Schema Migrations](plugin-schema-migrations.markdown)
+- [Creating your plugin](plugin-registration.markdown)
+- [Using plugin hooks](plugin-hooks.markdown)
+- [Override default application behaviors](plugin-overrides.markdown)
+- [Add schema migrations for plugins](plugin-schema-migrations.markdown)
+- [Add mail transports](plugin-mail-transports.markdown)
 
 Examples of plugins
 -------------------