Retail investors won't necessarily be able to replicate Ackman's success in Chipotle, but they can learn from it.Chipotle Mexican Grill (CMG -0.22%) has long been a primary holding of Pershing Square Capital Management, the fund managed by billionaire Bill Ackman. His fund first bought the stock during the second half of 2016, and it has earned considerable returns since that time.In total, Pershing Square has purchased 2.9 million shares worth approximately $1.1 billion. However, close to 2.2 million share ...