By Svea Herbst-Bayliss NEW YORK (Reuters) -Activist investor Scott Ferguson is joining the board of Performance Food Group, the company said on Tuesday, at a time the hedge fund is urging the food distributor to consider merging with rival US Foods Holding. Less than five weeks ago, Ferguson's Sachem Head Capital Management nominated four director candidates to Performance Food's board. Last week, Performance Food and US Foods signed an agreement to exchange sensitive financial information without violat ...