Hot off the Web! The Guardian, the “stalwart of free content,” will start charging users of its iOS 5-only iPad app as of this Friday, Jan. 13.To be clear, The Guardian does have a subscription service — users of its iPhone app can access three free articles per day before being asked to subscribe monthly. But the iPad app will start charging existing users (now receiving content under a free promotion with Channel 4 in the UK) for any content. New users will be allowed a seven-day trial before being facing a hard paywall asking for the monthly