CONTRIBUTING.md should mention git commit style

[jayvdb]

This repo uses emoji's in commit messages. Newcomers should be given hints on how to do that.

And old-comers also forget which emoji's to use for various situations.

There are a few so good time to make sure the current one is the best

• https://gitmoji.carloscuesta.me/
• https://github.com/slashsBin/styleguide-git-commit-message
• https://gist.github.com/parmentf/035de27d6ed1dce0b36a
• https://robinpokorny.github.io/git3moji/
• https://opensource.com/article/19/2/emoji-log-git-commit-messages

[jayvdb]

https://github.com/moremoban/moban/search?q=emoji&unscoped_q=emoji shows emoji isnt mentioned anywhere yet, except this issue now

[jayvdb]

According to coala/coala-bears#2524, https://github.com/slashsBin/styleguide-git-commit-message is the current style