Richardson joined Sunderland in 2007, going on to make 149 appearances for the Black Cats.
"I enjoyed it at Sunderland," he told safc.com. "I had a really good relationship with the fans. But I'd been there for five years and I felt like I needed a change. My family wanted to come back south as well.
"The Sunderland fans still talk about my free-kick against Newcastle United a few years ago. It was a good moment. I scored two free-kicks at Fulham once, but one got disallowed. Hopefully I can get involved in free-kicks here."