I was asked about Meltdown Cards today. These are business cards that parents can give out to people who offer those helpful parenting comments when their child in having a meltdown on the supermarket floor because they're not allowed to buy eight Peppa Pig DVDs.
They usually say some simple like "My child has autism and is having a meltdown. I'm sorry if this has caused you any distress, I can assure you I'm not enjoying it much either. If you'd like to learn more about autism, please visit the National Autistic Society at http://www.autism.org.uk".
Giving out a card can save you the job of telling the person what to do with their helpful parenting comments, and often causes them to apologise. If you Google "meltdown cards" lots of examples come up, personally I think simple and to-the-point works best. You can make them on your computer, or get 250 made from Vistaprint for around £5.