Clean up train README (#392)
* Update README.md
* Correct typo in README.md
* Update README.md
* Add intro to README.md
* Add TOC to train README.md
* Update TOC in train README.md
* Update scripts/train/README.md
* Update README.md
* add comment to benchmarking readme
* Update README.md
* Apply suggestions from code review
* Update scripts/train/README.md
* lint
* Update scripts/train/README.md
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Co-authored-by: Vitaliy Chiley <6439018+vchiley@users.noreply.github.com>
Co-authored-by: Vitaliy Chiley <vitaliy@mosaicml.com>
