update README.md wrt Cache-Control max-age

Change max-age from 10 years (315576000 seconds) to 1 year (31557600) (the actual maximum), as per https://github.com/carrierwaveuploader/carrierwave/issues/1466

Just as I was making this edit, I realized that it is even more convenient to say `365.day` (as opposed to `1.year`), as it enables people to easily change the number to 1.day or 7.day or 14.day or 30.day, etc., in accordance with what makes sense for their application.