Add documentation for user mentions

doc/index.markdown

@@ -45,6 +45,7 @@ Using Kanboard
 - [Create tasks by email](create-tasks-by-email.markdown)
 - [Subtasks](subtasks.markdown)
 - [Analytics for tasks](analytics-tasks.markdown)
+- [User mentions](mentions.markdown)
 
 ### Working with users and groups
 

doc/user-mentions.markdown

@@ -0,0 +1,17 @@
+User Mentions
+=============
+
+Kanboard offers the possibility to send notifications when someone is mentioned.
+
+If you need to get the attention of someone in a comment or in a task, use the @ symbol followed by their username.
+Kanboard will automatically suggest a list of users:
+
+![User Mention](screenshots/mention-autocomplete.png)
+
+- At the moment, only the task description and the comment text area have this feature enabled.
+- The user mentions works only during tasks and comments creation.
+- To be notified, mentioned users need to be a member of the project.
+- When someone is mentioned, this user will receive a notification.
+- The @username mention is linked to the public user profile.
+
+The notification is sent according to the user settings, it can be an email, a web notification or even a message on Slack/Hipchat/Jabber if you have installed the right plugins.