"I know." Maybe switch it up based on who the person is. Have they known you for a while? Then no shit you're stuttering, you have a stutter, they should know that by now. So I guess just keep it at "I know." If it's someone you've just met, then they don't know yet. Tell them something like, "I know, it's a speech disorder/impediment." Whichever route you go with, say it and continue with what you were saying.