[docs] CONTRIBUTING.md: specify bare infinitive instead of imperative

[wyattscarpenter]

This PR changes the git commit guidelines to say "Use the bare infinitive in the subject line" instead of "Use the imperative mood in the subject line"

Technically, git commit subject lines are in the bare infinitive, not the imperative, because they're merely unconjugated, rather than being a command. (They are identical in form, though, so it's understandable that they were confused here.)

Or, so I think, at least! For more information, you can see https://en.wikipedia.org/wiki/Bare_infinitive , https://en.wikipedia.org/wiki/Imperative_mood , or this blog post I wrote about this subject a while ago: https://wyattscarpenter.github.io/blog/git_commit_messages_are_in_the_bare_infinitive.txt

[sterliakov]

To my surprise I was familiar with both terms and the proposed wording is correct, but... IMO this is overly pedantic. There are a lot of contributors here who only speak very basic English. The term imperative should be familiar to them as it's just basic grammar, while bare infinitive may be less clear. I can easily imagine a commit/pr message "To prevent crash on a malformed input" because the author knows what infinitive means but never encountered bare infinitive before and therefore just skipped it...