Add the cache_only configuration option
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.
Merge pull request #1468 from abatko/patch-1

update README.md wrt Cache-Control max-age
to be future changes-proof don't rely on mime gem file extensions order
Merge pull request #1471 from vanburg/fix/don-t-rely-on-mime-gem-file-extensions-order

to be future changes-proof don't rely on mime gem file extensions order
Merge pull request #1456 from jeffkreeftmeijer/cache-only

Add the cache_only configuration option