Add doc for s3 access

[smcalilly]

Overview

• For #275, but doesn't close it because we need to script this process

Testing Instructions

• Review Google doc and edit for clarity
• Go through the steps outlined in the doc and add any missing information, flag any bad information, point out any confusing information, etc
• Add any suggestions for naming IAM and S3 conventions
• Review IAM policy and make sure it's accessing S3 the way we want it

[smcalilly]

@derekeder I made your change, thanks for the simpler step.