New York/Hong Kong - Warren Buffett has just joined the world's most exclusive club of the mega rich.The legendary American investor was worth $100 billion on Thursday, according to the Bloomberg Billionaire's Index. That puts him in the company of just five other men above that threshold. And he's now the sixth richest person in the world, ranking just behindFacebookCEO Mark Zuckerberg.The 90-year-old Buffett has added nearly $13 billion to his net worth this year as shares in his industrial a