Start to write some documentation about users management

docs/manage-users.markdown

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+Manage users
+============
+
+Type of users
+-------------
+
+Kanboard use a basic permission system, there is two kind of users:
+
+- Administrators
+- Regular/Standard users
+
+Administrator have access to everything. By example, they can add or remove projects.
+
+A standard user can't manage users and projects.
+
+Add a new user
+--------------
+
+To add a new user, you must be administrator.
+
+1. Go to the top menu **users**
+2. On the left, you have a link **New user**
+3. Fill the form and save
+
+When you create a new user, you have to specify those values:
+
+- **username**: This is the unique identifier of your user (login)
+- **password**: The password of your user must have at least 6 characters
+
+Eventually, you can choose to assign a default project to this user and allow him to be administrator.
+
+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 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 administrator.
+
+If you remove a specific user, **tasks assigned to this person will be unassigned** after the operation.