Backup safety and special teams standout Derrick Martin's season is over.  He was placed on injured reserve this morning by the Packers.  Martin suffered a significant knee sprain last Sunday in Washington, one of several Packers injured in that game.  To take his place on the active roster, the Packers signed linebacker Robert Francois.  The former Boston College linebacker first joined the Pack last year, spending the final four weeks of the season plus the playoff week on the practice squad.  He was released in the final cut this summer, brought back onto the practice squad for four of the past five weeks.  He's 6-2 and 225 poinds, originally signed by the Vikings as an undrafted free agent in May of 2009.   Green Bay also released linebacker Johnny Williams from the practice squad and added two players they worked out Tuesday.   Linebacker Mike Rivera, a first year player out of Kansas who spent all of last season on Tennessee's practice squad and safety Michael Greco, a rookie from Central Florida who was cut by the New York Giants at the end of training camp.