The following article is mentioned in our CONTRIBUTING.md file.
https://medium.com/@kurtisnusbaum/stacked-diffs-keeping-phabricator-diffs-small-d9964f4dcfa6
However, the recommended workflow does not actually work -- there does not seem to be a way to submit diffs in the middle of a rebase. We either need to write our own documentation, or you need to inform @schancel what he is doing wrong so he can help others and/or write documentation.