Palantir has been on a huge winning streak. Should you buy the stock before it's added to the S&P 500?Palantir (PLTR 13.65%) stock is making big gains again in Monday's trading. The company's share price was up 11.7% as of 11:30 a.m. ET, according to data from S&P Global Market Intelligence.Palantir is climbing in conjunction with news that the company's stock is going to be added to the S&P 500 index. Companies in the index have to meet criteria for profitability, valuation, trading volume, and other facto ...