I would keep it simple - After being married, you discovered that you and your wife are physically and emotionally incompatible, she has now abandoned the marriage and disappeared and that you plan to file for divorce or an anullment. The links provided by @Melancholic Mage above give you a great template and since you just filed last month, I would send the letter to the service center listed at the bottom of your I-797C receipt notice you received for the I-130.