Add comment addressing CORS to S3 config.
Include comment about CORS generically.

This has example CORS configurations for both S3 and GCS.
Remove S3 specific CORS comments
Reorganize CORS information into a new section.
s/it's/its and address the 'why' for CORS existing
Merge pull request #397 from jzelinskie/master

Add comment addressing CORS to S3 config.