Add a section to explain the different GitHub PR labels

[Mariatta]

Not sure if this is in the DevGuide already.
There should be an explanation somewhere of what the different labels mean, especially:

• skip issue
• skip news

It's not always clear especially to new contributors of what makes an issue "trivial", maybe that needs to be clarified somewhere, either in the devguide or in https://github.com/python/cpython/blob/master/.github/CONTRIBUTING.rst

[terryjreedy]

I notice that some core devs have a much broader idea of 'trivial' than others, and I think better agreement would be good.

The main use of 'skip news', other than no issue, is for follow-up PRs on the same issue that do not require editing the existing news blurb .

[ncoghlan]

https://docs.python.org/devguide/committing.html#what-s-new-and-news-entries has the details on when "skip news" is appropriate.

"skip issue" is similar but isn't currently documented anywhere: if it's a follow-up to an unreleased change, re-use the existing issue number (and NEWS entry), otherwise it should get a new issue, so the change can be associated with the corresponding release.

The main cases where skipping both an issue report and the NEWS entry is appropriate are:

• docs clarifications that don't make any changes to promised functionality (they just improve wording)
• fixes to comments

Even nominally internal refactorings should still get a NEWS entry, since they can introduce fresh bugs, and/or reveal cases where users were previously relying on implementation details that weren't covered by the test suite.

[Mariatta]

This would be useful info for the upcoming new Triagers team.

[Mariatta]

Some labels were explained here https://discuss.python.org/t/who-should-be-able-to-change-what-labels-on-github/2000