In fact, there's no official recommendation on this regard, that's why I created this PR trying to raise awareness. It's something I get confused about, people around the internet give different recommendations and I believe it would be a useful addition to the Guides.

The docs in this PR are merely a suggestion to get the conversation started.

@rafaelfranca:

1. You keep the highest version number of the two - as suggested in some of the resources I linked in the PR description -, and resolve the rest of the conflicts by hand?
2. Isn't it considered bad behavior to edit `db/schema.rb` by hand?
3. What kinds of things you had added or removed by running `rake db:migrate`? This is important, because those are the issues users need to watchout for while merging.