Added documentation for new notification API

Author: dennisdegreef

doc/notification.md

@@ -0,0 +1,38 @@
+## Notification API
+[Back to the navigation](index.md)
+
+Listing notifications and marking them as read.
+Wraps [GitHub Notification API](https://developer.github.com/v3/activity/notifications/).
+
+### List notifications
+
+```php
+$issues = $client->api('notification')->all();
+```
+
+Returns an array of unread notifications.
+
+### Include already read notifications, including participating, or since a certain date
+
+```php
+$includingRead = true;
+$participating = true;
+$since = new DateTime('1970/01/01');
+$issues = $client->api('notification')->all($includingRead, $participating, $since);
+```
+
+Returns an array of all notifications
+
+### Mark notifications as read
+
+```php
+$client->api('notification')->markRead();
+```
+
+or up until a certain date
+
+```php
+$client->api('notification')->markRead(new DateTime('2015/01/01'));
+```
+
+Marks all notifications as read up until the current date, unless a date is given