The wing-back has signed for the League Two club for an undisclosed fee on an 18-month deal after two-and-a-half seasons at Valley Parade.
Richards made 64 appearances for City with three goals and six assists but his progress was hampered by injury.
He began the promotion campaign strongly before his troubles began with a dislocated shoulder at Colchester in November.
It popped out again last February during a game at Wimbledon – and his comeback three weeks later at Bromley was cut short by a serious hamstring injury.
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Richards had surgery and spent the off-season recovering before signing a new one-year deal in July.
But he played only four times this term in the Vertu Trophy as well as City’s FA Cup first-round loss to Cheltenham.