[libc++] Add documentation about the libc++ review group

[ldionne]

Summary

This explains stuff that most contributors already know, but it's always
good to write down explicitly.

[philnik]

LGTM!

[• Quuxplusone]

libcxx/docs/Contributing.rst

51–52

If you upload the patch via the web interface, you should add "libc++" to the Reviewers field, and also add "libc++" to the Tags field. Adding "libc++" to the Subscribers field is unnecessary. (At least, that's what I've always done, and nobody's complained at me yet!)

60–62

It would be good to find a way to indicate who belongs to this group; otherwise
(1, the reactionary complaint) someone who considers themself a "frequent contributor with good understanding" may assume they are permitted to accept-as-libc++, when we don't want them to
(2, the more constructive complaint) someone who's definitely a libc++-accepter, such as myself, may see that a PR has been greenlit by another contributor, but I'm not sure whether that other contributor counts as a libc++-accepter, so I'm not sure whether I should accept-as-libc++ or not.

Maybe we don't want to put the names right here, either because it'll bit-rot (but maybe we should maintain the list!) or because we're too modest (but we shouldn't be!). But it'd sure be nice to do something explicit.

[Mordante]

LGTM modulo one small issue.

libcxx/docs/Contributing.rst

54

60–62

It would be good to find a way to indicate who belongs to this group; otherwise

+1 but not blocking for this review.

[ldionne]

Address comments.

[ldionne]

libcxx/docs/Contributing.rst

51–52

Above, we ask that folks upload patches from the command-line using arc. We could change that if you feel strongly about enabling that use case, but TBH I feel that we gain more from new contributors just using arc diff (and everything happening as it should) instead of trying to use the web interface. We've had hiccups with that before (and I'm even surprised it works nicely for you).

60–62

Oh, I guess I'm the only one aware about this, but the members can be seen here: https://reviews.llvm.org/project/members/64/

I'll add a link! And while we're at it -- it is worth mentioning that both @var-const and @philnik are now part of that group. Note that as we expand the group, my goal is to reduce the review bottlenecks without decreasing the code+tests quality or losing a coherent overall direction for the library. I think that can be achieved by people reviewing stuff and approving only when they are confident that they understand the area of the patch and its implications, and reaching out to the right people when necessary.

[ldionne]

@Quuxplusone LMK if you care strongly about the web interface review doc and I can add it (even though I would prefer to steer contributors towards arc diff).

libcxx/docs/Contributing.rst

- If you added a header:
   - Did you add it to ``include/module.modulemap``?
   - Did you add it to ``include/CMakeLists.txt``?
   - If it's a public header, did you add a test under ``test/libcxx`` that the new header defines ``_LIBCPP_VERSION``? See ``test/libcxx/algorithms/version.pass.cpp`` for an example. NOTE: This should be automated.
   - If it's a public header, did you update ``utils/generate_header_inclusion_tests.py``?
 
 - Did you add the relevant feature test macro(s) for your feature? Did you update the ``generate_feature_test_macro_components.py`` script with it?
 - Did you run the ``libcxx-generate-files`` target and verify its output?
 
+The review process
+==================
+
+After uploading your patch, you should see that the "libc++" review group is automatically
+added as a reviewer for your patch. Once the group is marked as having approved your patch,
+you can commit it. However, if you get an approval very quickly for a significant patch,
+please try to wait a couple of business days before committing to give the opportunity for
+other reviewers to chime in. If you need someone else to commit the patch for you, please
+mention it and provide your ``Name <email@domain>`` for us to attribute the commit properly.
+
+Note that the rule for accepting as the "libc++" review group is to wait for two members
+of the group to have approved the patch, excluding the patch author. This is not a hard
+rule -- for very simple patches, use your judgement. The `"libc++" review group <https://reviews.llvm.org/project/members/64/>`__
+consists of frequent libc++ contributors with a good understanding of the project's
+guidelines -- if you would like to be added to it, please reach out on Discord.
+
 Post-release check list
 =======================
 
 After branching for an LLVM release:
 
 1. Update ``_LIBCPP_VERSION`` in ``include/__config``
 2. Update the ``include/__libcpp_version`` file
 3. Update the version number in ``docs/conf.py``