Added sample config for cloudfront with S3

As per https://github.com/docker/docker-registry/issues/540#issuecomment-62754851 and discussion with @dmp42 on IRC submitting this pull request to illustrate how CloudFront  can be used in tandem with S3. This assumes that the user has CloudFront setup in front of S3 as caching layers. I was confused with regards to the documentation for this, I believe it should go in advanced.md if the collaborators can guide me with where to put the documentation I am happy to do so. Feedback is welcome.  
Instead of inheriting common, can you inherit "s3"? that will save you the S3-specific config in there