Create guide for S3 file storage using spring-cloud-aws

[gregturn]

Create a guide showing how to store, lookup, and retrieve files in S3 using http://cloud.spring.io/spring-cloud-aws/. Cloud native apps won't have local file storage nor have mounted fileshares.

Look at https://github.com/gregturn/spring-a-gram/tree/master/spring-a-gram-s3-fileservice as inspiration.