Docs: Update CONTRIBUTING.md #9542

Update CONTRIBUTING.md to document the different components.

Notably, trivial should only be used for PRs that do not change the code.

This is not really critical, but as jtimon points out there is a lot of confusion over "Trivial" tags for PR's.

I think that we'd save ourselves a lot of grief if we adopted a policy that was more in line with how people typically (and in many other projects) use "trivial" tags. Changing the contributing.md isn't going to change what people making their first PR do, as they may not have even read that. Usually, a trivial tag refers to a PR that either changes only document to fix typos or is a minor code change that is easily reviewable and has no visible behavior change.

For example, I would tag the code

int a = 1+1;

to

int a = 2;

as a Trivial change

but not something like

atomic_int a = 1;
++a;

to

atomic_int a = 0;
++a;
++a;

as there is a small behavior change if someone else (from another thread observing a) would see a difference. Other sorts of things I would tag as Trivial are namespace visibility, de-duplication, etc.

I think the other benefit of using "Trivial" to tag something that you think is trivial is that it is an assertion that the PR does not have any major affect, and if someone does see that it does have an effect then the PR should not be accepted (or at least re-thought).

For things that are "Trivial" in the sense @jnewbery is proposing, perhaps we can use either "DocOnly", "Typo", or "NonCode" as tags.

edit: I think ultimately it comes down to however @laanwj wants to manage it, but it would be nice to have a tag that means "Trivial" in the sense I am suggestion. ("Minor"? "Obvious"?)

@JeremyRubin - I don't feel strongly one way or the other. I just added this text because I've seen a few PRs where maintainers have said that trivial should just be used for non-code changes, and that didn't seem to be documented anywhere.

@JeremyRubin tend to agree mostly there These changes to the code are clearly trivial:

• Comment only
• Whitespace only
• Changes only variable names
• Typos in logging and messages

Mostly, anything that doesn't change generated executable could be regarded as trivial.

Not trivial:

• Refactors (change of function arguments, moves)
• Changes in behavior

Updated definition of trivial to reflect #9542 (comment)

Looks good to me now.

commits squashed

@JeremyRubin explained my perspective very well. I use the trivial tag to make code changes that should be trivial to review and don't change functionality, maybe even for things like #9176. If it takes more than a few minutes to review, I think "NACK this is not trivial" or "Nit remove the trivial label, this is not trivial". Anyway, should have left this comments some time ago and what has been merged is more formal.