Move English documentation to folder en_US

doc/en_US/notifications.markdown

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+Notifications
+=============
+
+Kanboard is able to send notifications through several channels:
+
+- Email
+- Web (List of unread messages)
+
+External plugins allow you to send notifications to Slack, Hipchat, Jabber or any chat system.
+
+Configuration
+-------------
+
+Each user must enable the notifications in their profile: **User Profile > Notifications**. It's disabled by default.
+
+To receive email notifications you need a valid email address in your profile and the application must be configured to send emails.
+
+![Notifications](screenshots/notifications.png)
+
+You can choose your favorite notification method:
+
+- Emails
+- Web (see below)
+
+For each project you are a member, you can choose to receive notifications for:
+
+- All tasks
+- Only for tasks assigned to you
+- Only for tasks created by you
+- Only for tasks created by you and assigned to you
+
+You can also select only some projects, by default it's all projects where you are a member.
+
+Web notifications
+-----------------
+
+Web notifications are available from the dashboard or from the icon at the top:
+
+![Web Notifications Icon](screenshots/web-notifications-icon.png)
+
+Notifications are shown in a list, so you can mark individual notification as read or everything.
+
+![Web Notifications](screenshots/web-notifications.png)
+
+In this way you can still get notified without having to receive emails.
+