Add documentation on transactional guarantees

[JoshRosen]

This pull request adds documentation on spark-redshift's transactional guarantees.

[codecov-io]

Current coverage is 88.99%

Merging #150 into master will not affect coverage as of 4990791

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[JoshRosen]

Since I'm going to fix this in #151, I should probably just update this now to move the current text into a pre-1.6.1 caveat / callout.

[yhuai]

Maybe we need to more specific on when eventual-consistency may have a impact because Amazon S3 buckets in all Regions provide read-after-write consistency for PUTS of new objects and eventual consistency for overwrite PUTS and DELETES. (https://aws.amazon.com/s3/faqs/)?

[JoshRosen]

I think that Amazon FAQ entry is really misleading / incomplete. My understanding of S3 read-after-write consistency is that

1. it doesn't apply to bucket listing operations (see https://docs.aws.amazon.com/AmazonS3/latest/dev/Introduction.html#ConsistencyModel), and
2. while all regions provide read-after-write consistency, you have to use certain endpoints to receive those guarantees (specifically, I don't think that the global endpoint provides this for us-standard buckets (I'd clarify this with Amazon if you're really curious about the specifics)).

I don't know why the Amazon documentation doesn't just spell this out more clearly.

[JoshRosen]

@yhuai, I've updated this to be organized slightly differently. PTAL.

[yhuai]

LGTM

[JoshRosen]

Alright, I'm going to merge this now and will create issues to work on the future improvements promised in the text.