Add commit message information to committer's guide

[emaste]

Summary

From an article I wrote, then through https://github.com/bsdimp/freebsd-git-docs/blob/main/commit.md

[imp]

Love it

[debdrup]

Looks good to me.

It's outside the scope of this review, but we might want to consider adding another column guide to the prepare-commit-msg hook, which indicates where 63 columns is located.

[emaste]

It's outside the scope of this review, but we might want to consider adding another column guide to the prepare-commit-msg hook, which indicates where 63 columns is located.

Good idea, we also need to sync the list of trailer tags between the template and this doc.