New section on how to remove a user password via console (#220)

* Add new section

Added "Remove password for user"-section.
Co-authored-by: Marcel Herrguth <github@thehomeofanime.de>

admin/console/working-on-users.rst

@@ -102,3 +102,16 @@ if needed.
 .. code-block:: ruby
 
    >> User.find_by(email: 'you@example.com').update!(password: 'your_new_password')
+   
+Remove password for user
+------------------------
+
+If you added a second authentication method (e.g. LDAP) after launch, there
+still may be a password in Zammad's own user management. In cases like that
+users will be able to login with their (local) Zammad password in addition to
+the credentials stored on the external authentication provider. Simply remove
+the password stored by Zammad.
+
+.. code-block:: ruby
+
+   >> User.find_by(email: 'you@example.com').update!(password: nil)