Move English documentation to folder en_US

doc/en_US/user-management.markdown

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+User Management
+===============
+
+Add a new user
+--------------
+
+To add a new user, you must be an administrator.
+
+1. From the dropdown menu in the top right corner, go to the menu **Users Management**
+2. On the top, you have a link **New local user** or **New remote user**
+3. Fill the form and save
+
+![New user](screenshots/new-user.png)
+
+When you create a **local user**, you have to specify at least those values:
+
+- **username**: This is the unique identifier of your user (login)
+- **password**: The password of your user must have at least 6 characters
+
+For **remote users**, only the username is mandatory.
+
+Edit users
+----------
+
+When you go to the **users** menu, you have the list of users, to modify a user click on the **edit link**.
+
+- If you are a regular user, you can change only your own profile
+- You have to be an administrator to be able to edit any users
+
+Remove users
+------------
+
+From the **users** menu, click on the link **remove**. This link is visible only if you are administrators.
+
+If you remove a specific user, **tasks assigned to this person will be unassigned** after the operation.