I was looking for instructions on backporting changes the other day and
wasn't able to find it anywhere. I updated the contrib guides based on
the disccusion in #6420, #6215 and #6447.
Add more info on how to write a good commit messages along with
example showing nicely formatted commit message.
Rails git history does not look too well when you try to figure out why
particular changes were introduced. We can do much better than that and
it's never too late to start.