Cotterill has expressed his delight at bringing in the versatile former Stevenage forward for an undisclosed fee.
The Robins boss told the club's official website: "He's someone we've been keeping tabs on for some time and I'm delighted we've been able to bring Luke on board.
"I've wanted to add a left-sided player to the squad and Luke can actually play in a variety of positions. He can play on the left, on the right and cut in, or play in a midfield three.
"In truth it's probably going to take six to 12 months to work out his best position, but that'll come by working with him in training.
"He's a good, young, exciting player. He's an excellent addition to the football club for the short term and long term."