Note on s3 documentation

[matthewchung74]

Not sure where to send feedback on the docs, but on this step here

http://keystonejs.com/docs/configuration/#services-amazons3

With my newly created bucket, I had to add the S3_REGION environmental variable as well. Without it, I was getting errors.

[carlafranca]

I would add to it the caution when choosing the region when creating the S3.
The "Signature Version(s) support" (on keystonejs) seems to only support version 2.
I tried to use Frankfurt but It didn't work. "version 4 only"
http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region

[mxstbr]

keystonejs/keystonejs-site#106