If you have a way to contact them through email in advance you can introduce yourself a little bit. Say something along the lines of... Thank you for the opportunity to interview with McDonalds. Then follow with something like...I want to inform you that I have a chronic speech impediment. This will not detract from my work ethic or any responsibilities with the job. It does however occasionally present itself during times of anxiety. I think this job would be a perfect fit for me. I am however a bit nervous to the hiring process and wanted to let you know in advance.....Just be honest. Don't go overboard. But be honest. It leaves it open-ended, but still obvious enough. Also, be articulate and write in complete sentences. There are still crazies out there who think a speech impediment is the equivalent to a cognitive disorder. Then when you show up to the interview, if it happens, it happens. But everyone involved will know and not affect the outcome.