[Plugin: simpleSAMLphp Authentication] Please explain how to configure an eduPersonEntitlement

As far as I know that is currently meant as specifying the value the attribute “eduPersonEntitlement” should have to allow the user admin prilvileges.

This is not strictly needed to “promote” just one user to admin, as that can be done when following the installation instructions (two browser, one logged in before activating the plugin, the other logged in with the to-be admin account). You can then with your admin account promote the user that logged in through simplesamlphp-authentication to admin.