Rihanna’s little-known philanthropy has been giving away millions for more than a decade. Her ‘uncommon’ strategy is a case study for other celebrities to follow

Rihanna is accustomed to defying convention. The nine-time Grammy winner has turned her wide-ranging string of hits, including “Umbrella” and “Work,” into a business empire worth an estimated $1.4 billion, placing her high on last year's Forbes list of the richest “self-made” American women. The…