Billionaire investor Bill Ackman is planning to take his Pershing Square management company (PS) public. But in doing so, Ackman is taking an unusual route: He is also starting a new fund, Pershing Square USA (PSUS), and if you want to get in on the Pershing Square management company’s initial public offering (IPO), the only way to do so is to buy shares in the new fund first. Here’s what you need to know about Pershing Square’s IPO: ## Pershing’s combined IPO When announcing its intention to go public, Pershing Square Inc. also announced that it will launch a new fund called Pershing Square USA (PSUS), and investors in the new fund will receive a set number of shares in Pershing Square Inc. (PS).…