Added user manual

docs/getting-started.md

@@ -29,3 +29,26 @@ cd ./moodle-auth_token
 ```
 
 ## User's Manual
+
+An authentication plug-in is a method of handling user authentication to Moodle when users log into your site.
+You can have one or more methods as the same time enabled on your site, but each user can only use one method of authentication at a time.
+
+You must enable and configure the method, as you need for your users in `Site administration` > `Plugins` > `Authentication` > `Manage authentication`.
+Note that the order of processing on this page does matter and after `manual` and `nologin`, you should next put the method that most users will have.
+
+![Manage authentication](screenshots/manageauths.jpg)
+
+The authentication plug-in has no configuration.
+This means that the third-party plug-in is responsible for setting the keys.
+
+**Note**:
+Users will not receive any error or other message when they try to log in but it simply will not allow them in.
+
+To authenticate a user, create a link for it:
+
+```Text
+/login/index.php?token=secret
+```
+
+The authentication plug-in does not add graphical forms to the user.
+This means that custom forms are created by developers for a particular task.