SubmittingPatches: encourage trailer use for substantial help

Trailers beyond the mandatory s-o-b are regularly used based on my last two years of reading the mailing list. Moreover, reviewers might encourage it.[1] This is also in line with the project crediting both commit authors and people mentioned in trailers each release; “Nobody is THE one making contribution”.[2] Adding trailers is already encouraged, but in the section `send-patches`. Let’s replace “If you like” with outright encouragement in this section so that all trailer discussion (except s-o-b; see `sign-off` section) is contained in this section; a link to from `send-patches` makes this information equally visible. Now we need to make a heading for `commit-trailers` in order for the HTML output to make sense. At the same time, it is important to temper this recommendation to a significant enough contribution; in my experience beginners can be eager to add a trailer for everyone who replies with an action point that is followed up on. Let’s also spell out that these trailers should follow the Git author/ committer format. One might naturally just write the name, but in that case it will not be picked up by: git shortlog --group=trailer:<key> and normalization via `.mailmap` will not work. Also introduce the list of common trailers as such. Granted, this is already implied by the later paragraph about “create your own trailer”, so this just frontloads this information. † 1: https://lore.kernel.org/git/CAP8UFD0POvYDgGtEx8GBhvKkd8XzzWQsy8XxAKL9M3+uz3ka+w@mail.gmail.com/#:~:text=for%20at%20least † 2: https://lore.kernel.org/git/xmqqzh248sy0.fsf@gitster.c.googlers.com/ Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>