Section 12.  The compensation or method of fixing the compensation of an elected official of any local governmental subdivision which operates under a home rule charter or plan of government, as provided in Sections 4 and 5 of this Article, shall be provided in its charter.  The compensation or method of fixing the compensation of an elected official of any other local governmental subdivision shall be provided by law.  Compensation of a local official shall not be reduced during the term for which he is elected.