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 issue #1466 -- though 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.