Suggestion for S3 usage instructions

[markmeulemans]

It wasn't a big deal or hard to figure out, but it may be nice to mention the aws-s3 gem needs to be installed to use the S3 feature. More of a suggestion for the documentation than a real issue.

Thanks, and this is awesome by the way!

[kbingman]

Thanks. I just changed it and will push it with some other changes today